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THE  HEAVENS  ABOVE 


POPULAR    HANDBOOK    OF   ASTRONOMY. 


BY 


J.    A.    GILLET, 

PROFESSOR  OF  PHYSICS   IN  THE  NORMAL   COLLEGE   OF  THE  CITY  OF   NEW  YORK, 


W.    J.    ROLFE, 

FORMERLY   HEAD   MASTER  OF  THE   HIGH   SCHOOL, 
CAMBRIDGE,    MASS. 


WITH   SIX  LITHOGRAPHIC  PLATES  AND  FOUR   HUNDRED 
AND  SIXTY  WOOD  ENGRAVINGS. 


POTTER,    AINSWORTH,    &    CO., 

NEW  YORK   AND   CHICAGO. 
1882. 


COPYRIGHT  BY 

J.  A.  GILLET  AND  W.  J.  ROLFE, 
1882. 


jFranfcltn  Press: 

RAND,   AVERY,   AND   COMPANY, 
BOSTON. 


PREFACE. 


IT  has  been  the  aim  of  the  authors  to  give  in 
this  little  book  a  brief,  simple,  and  accurate  account 
of  the  heavens  as  they  are  known  to  astronomers 
of  the  present  day.  It  is  believed  that  there  is 
nothing  in  the  book  beyond  the  comprehension  of 
readers  of  ordinary  intelligence,  and  that  it  contains 
all  the  information  on  the  subject  of  astronomy  that 
is  needful  to  a  person  of  ordinary  culture.  The 
authors  have  carefully  avoided  dry  and  abstruse 
mathematical  calculations,  yet  they  have  sought  to 
make  clear  the  methods  by  which  astronomers  have 
gained  their  knowledge  of  the  heavens.  The  various 
kinds  of  telescopes  and  spectroscopes  have  been 
described,  and  their  use  in  the  study  of  the  heavens 
has  been  fully  explained. 

The  cuts  with  which  the  book  is  illustrated  have 
been  drawn  from  all  available  sources  ;  and  it  is  be- 
lieved that  they  excel  in  number,  freshness,  beauty, 
and  accuracy  those  to  be  found  in  any  similar  work. 
The  lithographic  plates  are,  with  a  single  exception, 
reductions  of  the  plates  prepared  at  the  Observa- 

258222 


IV  PREFACE. 

tory  at  Cambridge,  Mass.  The  remaining  litho- 
graphic plate  is  a  reduced  copy  of  Professor  Lang- 
ley's  celebrated  sun-spot  engraving.  Many  of  the 
views  of  the  moon  are  from  drawings  made  from 
the  photographs  in  Carpenter  and  Nasmyth's  work 
on  the  moon.  The  majority  of  the  cuts  illustrating 
the  solar  system  are  copied  from  the  French  edition 
of  Guillemin's  "Heavens."  Most  of  the  remainder 
are  from  Lockyer's  "  Solar  Physics,"  Young's  "  Sun," 
and  other  recent  authorities.  The  cuts  illustrating 
comets,  meteors,  and  nebulae,  are  nearly  all  taken 
from  the  French  editions  of  Guillemin's  "  Comets  " 
and  Guillemin's  "  Heavens." 


CONTENTS. 


PAGE 

I.  THE   CELESTIAL   SPHERE       .       .       .       .  3 
II.  THE   SOLAR   SYSTEM       .       .       .       .       .       .41 

I.  THEORY   OF   THE    SOLAR   SYSTEM         .        .  41 

THE  PTOLEMAIC  SYSTEM 41 

THE  COPERNICAN  SYSTEM 44 

TYCHO  BRAKE'S  SYSTEM 44 

KEPLER'S  SYSTEM 44 

THE  NEWTONIAN  SYSTEM   .        .        .        .        .        .48 

II.  THE   SUN   AND   PLANETS 53 

I.  THE  EARTH 53 

FORM  AND  SIZE 53 

DAY  AND  NIGHT 57 

THE  SEASONS 64 

TIDES 68 

THE  DAY  AND  TIME 74 

THE  YEAR 78 

WEIGHT  OF  THE  EARTH  AND  PRECESSION        .  83 

II.  THE  MOON 86 

DISTANCE,  SIZE,  AND  MOTIONS  ....  86 

THE  ATMOSPHERE  OF  THE  MOON  .        .        .        .109 

THE  SURFACE  OF  THE  MOON      ....  114 

III.  INFERIOR  AND  SUPERIOR  PLANETS    .        .                .  130 

INFERIOR  PLANETS 130 

SUPERIOR  PLANETS 134 

vii 


Vlll  CONTENTS. 

PAGE 

IV.  THE  SUN 140 

I.  MAGNITUDE  AND  DISTANCE  OF  THE  SUN     .        .  140 
II.  PHYSICAL   AND    CHEMICAL   CONDITION    OF  THE 

SUN 149 

PHYSICAL  CONDITION  OF  THE  SUN    .       .       .149 

THE  SPECTROSCOPE 152 

SPECTRA         . 1 58 

CHEMICAL  CONSTITUTION  OF  THE  SUN         .  164 

MOTION  AT  THE  SURFACE  OF  THE  SUN    .        .  168 

III.  THE  PHOTOSPHERE  AND  SUN-SPOTS  .       .       .  175 

THE  PHOTOSPHERE 175 

SUN-SPOTS 179 

IV.  THE  CHROMOSPHERE  AND  PROMINENCES      .        .  196 
V.  THE  CORONA 204 

V.  ECLIPSES 210 

VI.  THE  THREE  GROUPS  OF  PLANETS  .        .        .        .  221 

I.  GENERAL  CHARACTERISTICS  OF  THE  GROUPS       .  221 

II.  THE  INNER  GROUP  OF  PLANETS       ...  225 

MERCURY 225 

VENUS -  ...  230 

MARS ".  235 

III.  THE  ASTEROIDS 241 

IV.  OUTER  GROUP  OF  PLANETS    .....  244 

JUPITER 244 

THE  SATELLITES  OF  JUPITER  ....  250 

SATURN 255 

THE  PLANET  AND  HIS  MOONS        .        .        .255 

THE  RINGS  OF  SATURN       ....  261 

URANUS 269 

NEPTUNE 271 

VII.  COMETS  AND  METEORS 274 

I.  COMETS 274 

GENERAL  PHENOMENA  OF  COMETS    .        .       .  274 

MOTION  AND  ORIGIN  OF  COMETS  ...  281 

REMARKABLE  COMETS 290 


CONTENTS.  ix 

I.  COMETS,  continued.  PAGE 

CONNECTION  BETWEEN  METEORS  AND  COMETS,  300 
PHYSICAL    AND    CHEMICAL  CONSTITUTION    OF 

COMETS 3r4 

II.  THE  ZODIACAL  LIGHT 318 

III.  THE   STELLAR   UNIVERSE        .       .       .       .322 

I.  GENERAL  ASPECT  OF  THE  HEAVENS        ...  322 

II.  THE  STARS     . 33o 

THE  CONSTELLATIONS 330 

CLUSTERS 350 

DOUBLE  AND  MULTIPLE  STARS     ....  355 

NEW  AND  VARIABLE  STARS 358 

DISTANCE  OF  THE  STARS 364 

PROPER  MOTION  OF  THE  STARS       ....  365 
CHEMICAL  AND  PHYSICAL  CONSTITUTION  OF  THE 

STARS ^71 

III.  NEBULA 373 

CLASSIFICATION  OF  NEBUL/E        ....  373 

IRREGULAR  NEBULAE -,76 

SPIRAL  NEBULAE 384 

THE  NEBULAR  HYPOTHESIS 39! 

IV.  THE  STRUCTURE  OF  THE  STELLAR  UNIVERSE       .  396 


ASTRONOMY. 


ASTRONOMY. 


i. 

THE   CELESTIAL   SPHERE. 

i .  The  Sphere.  —  A  sphere  is  a  solid  figure  bounded  by 
a  surface  which  curves  equally  in  all  directions  at  every 
point.  The  rate  at  which  the  surface  curves  is  called  the 
curvature  of  the  sphere.  The  smaller  the  sphere,  the  greater 
is  its  curvature.  Every  point  on  the  surface  of  a  sphere  is 
equally  distant  from  a  point  within,  called  the  centre  of 
the  sphere.  The  circumference  of  a  sphere  is  the  distance 
around  its  centre.  The  diameter  of  a  sphere  is  the  dis- 
tance through  its  centre.  The  radius  of  a  sphere  is  the 
distance  from  the  surface  to  the  centre.  The  surfaces  of 
two  spheres  are  to  each  other  as  the  squares  of  their  radii 
or  diameters ;  and  the  volumes  of  two  spheres  are  to  each 
other  as  the  cubes  of  their  radii  or  diameters. 

Distances  on  the  surface  of  a  sphere  are  usually  denoted  in 
degrees.  A  degree  is  3-^  of  the  circumference  of  the  sphere. 
The  larger  a  sphere,  the  longer  are  the  degrees  on  it. 

A  curve  described  about  any  point  on  the  surface  of  a 
sphere,  with  a  radius  of  uniform  length,  will  be  a  circle. 
As  the  radius  of  a  circle  described  on  a  sphere  is  a  curved 
line,  its  length  is  usually  denoted  in  degrees.  The  circle 
described  on  the  surface  of  a  sphere  increases  with  the 
length  of  the  radius,  until  the  radius  becomes  90°,  in  which 
case  the  circle  is  the  largest  that  can  possibly  be  described 

3 


ASTRONOMY. 


on  the  sphere.  The  largest  circles  that  can  be  described  on 
the  surface  of  a  sphere  are  called  great  circles,  and  all  other 
circles  small  circles. 

Any  number  of  great  circles  may  be  described  on  the  sur- 
face of  a  sphere,  since  any  point  on  the  sphere  may  be  used 
for  the  centre  of  the  circle.  The  plane  of  every  great  circle 
passes  through  the  centre  of  the  sphere,  while  the  planes  of 
all  the  small  circles  pass  through  the  sphere  away  from  the 
centre.  All  great  circles  on  the  same  sphere  are  of  the  same 
size,  while  the  small  circles  differ  in  size  according  to  the  dis- 
tance of  their  planes  from  the  centre  of  the  sphere.  The  far- 
ther the  plane  of  a  circle  is  from  the  centre  of  the  sphere,  the 
smaller  is  the  circle. 

By  a  section  of  a  sphere  we  usually  mean  the  figure  of  the 
surface  formed  by  the  cutting  :  by  'A.  plane  section  we  mean  one 

whose  surface  is  plane.  Every 
plane  section  of  a  sphere  is 
a  circle.  When  the  section 
passes  through  the  centre  of 
the  sphere,  it  is  a  great 
circle:  in  every  other  case 

^ie  sect'on  's  a  small  circle. 
Thus,  AX  and  ^/>  (Fig.  n 
are  small  circles,  and  M ' M' 
and  6" TV  are  large  circles. 

In  a  diagram  representing 
a  sphere  in  section,  all  the 
circles  whose  planes  cut  the 
section  are  represented  by 
straight  lines.  Thus,  in  Fig.  2.  \ve  have  a  diagram  representing 
in  section  the  sphere  of  Fig.  i.  The  straight  lines  AN,  SB. 
MM',  and  S N,  represent  the  corresponding  circles  of  Fig.  \. 

The  axis  of  a  sphere  is  the  diameter  on  which  it  rotates. 
The  poles  of  a  sphere  are  the  ends  of  its  axis.  Thus,  sup- 
posing the  spheres  of  Figs.  \  and  2  to  rotate  on  the  diame- 
ter PP',  this  line  would  be  called  the  axis  of  the  sphere, 
and  the  points  P  and  P'  the  poles  of  the  sphere.  A  great 


ASTRONOMY. 


circle,   MM1 ',  situated    half  way  between    the    poles    of  a 
sphere,  is  called  the  equator  of  the  sphere. 

Every  great  circle  of  a  sphere  has  two  poles.  These  are 
the  two  points  on  the  sur- 
face of  the  sphere  which  lie 
90°  away  from  the  circle. 
The  poles  of  a  sphere  are 
the  poles  of  its  equator. 

2.  The  Celestial  Sphere. 
—  The  heavens  appear  to 
have  the  form  of  a  sphere, 
whose  centre  is  at  the  eye 
of  the  observer  ;  and  all  the 
stars  seem  to  lie  on  the  sur- 
face of  this  sphere.  This 
form  of  the  heavens  is  a 
mere  matter  of  perspective.  The  stars  are  really  at  very 
unequal  distances  from  us  ;  but  they  are  all  seen  project- 
ed upon  the  celestial 
sphere  in  the  direc- 
tion in  which  they 
happen  to  lie.  Thus, 
suppose  an  observer 
situated  at  C  (Fig.  3). 
stars  situated  at  a.  b. 
d,  e,  /,  and  g,  would 
be  projected  upon  the 
sphere  atA,&,  D,  E. 
F,  and  G,  and  would 
appear  to  lie  on  the 
surface  of  the  heav- 
ens. 

3 .  The  Horizon .  — 
Only  halt'  of  the  celestial  sphere  is  visible  at  a  time.  The 
plane  that  separates  the  visible  from  the  invisible  portion  is 


ASTRONOMY. 


called  the  horizon.  This  plane  is  tangent  to  the  earth  at 
the  point  of  observation,  and  extends  indefinitely  into  space 
in  every  direction.  In  Fig.  4,  E  represents  the  earth,  O  the 

point  of  observation,  and  SN 
the  horizon.  The  points  on 
the  celestial  sphere  directly 
above  and  below  the  observer 
are  the  poles  of  the  horizon. 
They  are  called  respectively 
the  zenith  and  the  nadir.  No 
two  observers  in  different 
parts  of  the  earth  have  the 
same  horizon ;  and  as  a  per- 
son moves  over  the  earth  he 
carries  his  horizon  with  him. 


Fig.   4- 


The  dome  of  the  heavens  appears  to  rest  on  the  earth, 
as  shown  in  Fig.  5.  This  is  because  distant  objects  on 
the  earth  appear  pro- 
jected against  the 
heavens  in  the  di- 
rection of  the  hori- 
zon. 

The  sensible  hori- 
zon is  a  plane  tan- 
gent to  the  earth 
at  the  point  of  ob- 
servation. The  ra- 
tional horizon  is  a 
plane  parallel  with 
the  sensible  horizon, 
and  passing  through 
the  centre  of  the 


Fig.  5- 


earth.  As  it  cuts  the  celestial  sphere  through  the  cen- 
tre, it  forms  a  great  circle.  SN  (Fig.  6)  represents 
the  sensible  horizon,  and  S'  N'  the  rational  horizon. 


ASTRONOMY. 


Although  these  two  horizons  are  really  four  thousand  miles 
apart,  they  appear  to  meet  at  the  distance  of  the  celestial 
sphere  ;  a  line  four  thousand  miles  long  at  the  distance  of 
the  celestial  sphere  becom- 
ing a  mere  point,  far  too 
small  to  be  detected  with 
the  most  powerful  tele- 
scope. 

4.  Rotation  of  the  Celes- 
tial Sphere.  —  It  is  well 
known  that  the  sun  and  the 
majority  of  the  stars  rise  in 
the  east,  and  set  in  the  west. 
In  our  latitude  there  are  Fig.  6. 

certain  stars  in  the  north  which  never  disappear  below  the 
horizon.  These  stars  are  called  the  circumpolar  stars.  A 
close  watch,  however,  reveals  the  fact  that  these  all  appear 
to  revolve  around  one  of  their  number  called  the  pole  star, 

in  the  direction  indi- 
cated by  the  arrows  in 
Fig.  7.  In  a  word,  the 
whole  heavens  appear 
to  rotate  once  a  day. 
from  east  to  west, 
about  an  axis,  which 
is  the  prolongation  of 
the  axis  of  the  earth. 
The  ends  of  this  axis 
are  called  the  poles 
of  the  heavens  ;  and 
-  ?•  the  great  circle  of  the 

heavens,  midway  between  these  poles,  is  called  the  celestial 
equator,  or  the  equinoctial.  This  rotation  of  the  heavens 
is  apparent  only,  being  due  to  the  rotation  of  the  earth 
from  west  to  east. 


ASTRONOMY. 


5.  Diurnal  Circles.  —  In  this  rotation  of  the  heavens,  the 
stars  appear  to  describe  circles  which  are  perpendicular  to 
the  celestial  axis,  and  parallel  with  the  celestial  equator. 
These  circles  are  called  diurnal  circles.  The  position  of 

the  poles  in  the  heavens 
and  the  direction  of  the 
diurnal  circles  with  reference 
to  the  horizon,  change  with 
the  position  of  the  observer 
|jr  on  the  earth.  This  is  owing 
to  the  fact  that  the  horizon 
changes  with  the  position  of 
the  observer. 

When  the  observer  is  north 
of  the  equator,  the  north 
pole  of  the  heavens  is  ele- 
vated above  the  horizon,  and  the  south  pole  is  depressed 
below  it,  and  the  diurnal  circles  are  oblique  to  the  horizon, 
leaning  to  the  south.  This  case  is  represented  in  Fig.  8,  in 
which  P  P'  represents  the 
celestial  axis,  E  Q  the  celes- 
tial equator,  6"  N  the  horizon, 
and  ab,  c  TV,  de,  fg,  Sh, 
kl,  diurnal  circles.  O  is  the 
point  of  observation,  Z  the 
zenith,  and  Z'  the  nadir. 

When  the  observer  is  south 
of  the  equator,  as  at  O  in 
Fig.  9,  the  south  pole  is 
elevated,  the  north  pole  de- 
pressed, and  the  diurnal  cir- 
cles are  oblique  to  the  horizon,  leaning  to  the  north.  When 
the  diurnal  circles  are  oblique  to  the  horizon,  as  in  Figs.  8 
and  9,  the  celestial  sphere  is  called  an  oblique  sphere. 

When  the  observer  is  at  the  equator,  as  in   Fig.  10,  the 


ASTRONOMY. 


poles  of  the  heavens  are  on  the  horizon,  and  the  diurnal 

circles  are  perpendicular  to  the  horizon. 

When  the  observer  is  at  one  of  the  poles,  as  in  Fig.  n, 

the    poles    of    the    heavens 

are   in   the   zenith    and    the 

nadir,  and   the   diurnal   cir- 

cles   are  parallel  with    the 

horizon. 

6.  Elevation  of  the  Pole 

and  of  the  Equinoctial.  — 

At    the    equator   the    poles 

of   the    heavens  lie   on   the 

horizon,    and    the    celestial 

equator  passes   through   the 

zenith.     As  a  person  moves 

north  from  the  equator,  his 

zenith  moves  north  from  the  celestial  equator,  and  his  hori- 

zon moves  down  from  the  north  pole,  and  up  from  the  south 

pole.     The  distance  of  the  zenith  from  the  equinoctial,  and 

of  the  horizon  from  the  celes- 
tial poles,  will  always  be  equal 
to  the  distance  of  the  observ- 
er from  the  equator.  In  other 
words,  the  elevation  of  the 
pole  is  equal  to  the  latitude 
of  the  place.  In  Fig.  12,  O 
is  the  point  of  observation. 
/,  the  zenith,  and  SN  the 
horizon.  N  P,  the  elevation 
of  the  pole,  is  equal  to  ZE, 
the  distance  of  the  zenith 
from  the  equinoctial,  and  to 


the  distance  of  O  from  the  equator,  or  the  latitude  of  the 
place. 

Two  angles,  or  two  arcs,  which  together  equal   90°,  are 


10 


ASTRONOMY. 


said  to  be  complements  of  each  other.  ZE  and  E  S  in 
Fig.  1 2  are  together  equal  to  90°  :  hence  they  are  comple- 
ments of  each  other.  /,  R  is  equal  to  the  latitude  of  the 

place,  and  E  S  is  the  eleva- 
tion of  the  equinoctial  above 
the  horizon  :  hence  the  ele- 
vation of  the  equinoctial  is 
equal  to  the  complement,  of 
|  if  the  latitude  of  the  place. 

Were  the  observer  south 
of  the  equator,  the  zenith 
would  be  south  of  the  equi- 
noctial, and  the  south  pole 
of  the  heavens  would  be  the 
elevated  pole. 

7.  Four  Sets  of  Stars. — At  most  points  of  observation 
there  are  four  sets  of  stars.     These  four  sets  are  shown  in 

Fig-  13- 

(i)  The  stars  in 
the  neighborhood  of 
the  elevated  pole 
never  set.  It  will 
be  seen  from  Fig. 
13,  that  if  the  dis- 
tance of  a  star  from 
the  elevated  pole 
does  not  exceed  the 
elevation  of  the  pole, 
or  the  latitude  of 
the  place,  its  diurnal 
circle  will  be  wholly 
above  the  horizon. 
As  the  observer  approaches  the  equator,  the  elevation  of 
the  pole  becomes  less  and  less,  and  the  belt  of  circumpolar 
stars  becomes  narrower  and  narrower :  at  the  equator  it 


Fig.  13- 


ASTRONOMY.  1 1 

disappears  entirely.  As  the  observer  approaches  the  pole. 
the  elevation  of  the  pole  increases,  and  the  belt  of  circum- 
polar  stars  becomes  broader  and  broader,  until  at  the  pole 
it  includes  half  of  the  heavens.  At  the  poles,  no  stars  rise 
or  set,  and  only  half  of  the  stars  are  ever  seen  at  all. 

(2)  The  stars  in  the  neighborhood  of  the  depressed  pole 
never  rise.     The  breadth  of  this  belt  also  increases  as  the 
observer  approaches  the  pole,  and  decreases  as  he  approaches 
the  equator,  to  vanish  entirely  when  he  reaches  the  equator. 
The  distance  from  the  depressed  pole  to  the  margin  of  this 
belt  is  always  equal  to  the  latitude  of  the  place. 

(3)  The  stars  in  the  neighborhood  of  the  equinoctial,  on 
the  side  of  the  elevated  pole,  set,  but  are  above  the  horizon 
longer  than  they  are  below  it.     This  belt  of  stars  extends 
from  the  equinoctial   to   a  point  whose  distance  from   the 
elevated  pole  is  equal  to  the  latitude  of  the  place  :  in  other 
words,  the  breadth  of  this  third  belt  of  stars  is  equal  to 
the  complement  of  the  latitude  of  the  place.     Hence  this 
belt  of  stars  becomes  broader  and  broader  as  the  observer 
approaches  the  equator,  and  narrower  and  narrower  as  he 
approaches  the  pole.     However,  as  the  observer  approaches 
the  equator,  the  horizon  comes  nearer  and  nearer  the  celes- 
tial axis,  and  the  time  a  star  is  below  the  horizon  becomes 
more  nearly  equal  to  the  time  it  is  above  it.     As  the  observ- 
er approaches  the  pole,  the  horizon  moves  farther  and  far- 
ther from  the  axis,  and   the   time  any  star  of  this  belt  is 
below  the  horizon  becomes  more  and  more  unequal  to  the 
time  it  is  above  it.     The  farther  any  star  of  this  belt  is  from 
the  equinoctial,  the  longer  the  time  it  is  above  the  horizon. 
and  the  shorter  the  time  it  is  below  it. 

(4)  'Hie    stars  which    are    in    the    neighborhood  of  the 
equinoctial,  on  the  side  of  the  depressed  pole,  rise,  but  are 
below  the  horizon  longer  than  they  are  above  it.     The  width 
of  this  belt  is  also  equal  to  the  complement  of  the  latitude 
of  the  place.     The  farther  any  star  of  this  belt  is  from  the 


12 


ASTRONOMY. 


equinoctial,  the  longer  time  it  is  below  the  horizon,  and  the 
shorter  time  it  is  above  it ;  and,  the  farther  the  place  from 
the  equator,  the  longer  every  star  of  this  belt  is  below  the 
horizon,  and  the  shorter  the  time  it  is  above  it. 

At  the  equator  every  star 
is  above  the  horizon  just 
half  of  the  time ;  and  any 
star  on  the  equinoctial  is 
above  the  horizon  just  half 
of  the  time  in  every  part  of 
the  earth,  since  the  equinoc- 
tial and  horizon,  being  great 
circles,  bisect  each  other. 

8.  Vertical  Circles.  — 
Great  circles  perpendicular 
to  the  horizon  are  called  ver- 
tical circles.  All  vertical  circles  pass  through  the  zenith  and 
nadir.  A  number  of  these  circles  are  shown  in  Fig.  14. 
in  which  SE N  ^represents  the  horizon,  and  Z  the  zenith. 

The  vertical  circle  which 
passes  through  the  north  and 
south  points  of  the  horizon 
is  called  the  meridian;  and 
the  one  which  passes  through 
the  east  and  west  points,  the  * 
prime  vertical.  These  two 
circles  are  shown  in  Fig.  15  ; 
SZN  being  the  meridian, 
and  EZWti\z  prime  verti- 
cal. These  two  circles  are 
at  right  angles  to  each  other,  Fig.  15. 

or  90°  apart ;  and  consequently  they  divide  the  horizon  into 
four  quadrants. 

9.  Altitude  and  Zenith    Distance.  —  The    altitude   of  a 
heavenly  body  is   its   distance   above   the   horizon,  and  its 


ASTRONOMY.  13 

zenith  distance  is  its  distance  from  the  zenith.  Both  the 
altitude  and  the  zenith  distance  of  a  body  are  measured  on 
the  vertical  circle  which  passes  through  the  body.  The  alti- 
tude and  zenith  distance  of  a  heavenly  body  are  comple- 
ments of  each  other. 

10.  Azimuth  and  Amplitude.  — Azimuth  is  distance  meas- 
ured east  or  west   from   the   meridian.     When   a  heavenly 
body  lies  north  of  the  prime  vertical,  its  azimuth  is  meas- 
ured from  the   meridian   on  the   north  ;   and,  when  it  lies 
south  of  the  prime  vertical,  its  azimuth  is  measured  from  the 
meridian  on  the  south.     The  azimuth  of  a  body  can,  there- 
fore, never  exceed  90°.     The  azimuth  of  a  body  is  the  angle 
which  the  plane  of  the  vertical  circle    passing   through   it 
makes  with  that  of  the  meridian. 

The  amplitude  of  a  body  is  its  distance  measured  north 
or  south  from  the  prime  vertical.  The  amplitude  and  azi- 
muth of  a  body  are  complements  of  each  other. 

11.  Alt- azimuth  Instrument.  —  An   instrument  for  meas- 
uring the  altitude  and  azimuth  of  a  heavenly  body  is  called 
an  alt-azimuth  instrument.     One  form  of  this  instrument  is 
shown  in  Fig.    16.     It   consists    essentially  of  a    telescope 
mounted  on  a  vertical  circle,  and  capable  of  turning  on  a 
horizontal  axis,  which,  in  turn,  is  mounted  on  the  vertical 
axis  of  a   horizontal   circle.     Both  the   horizontal  and  the 
vertical  circles  are  graduated,  and  the  horizontal  circle'  is 
placed  exactly  parallel  with  the  plane  of  the  horizon. 

When  the  instrument  is  properly  adjusted,  the  axis  of  the 
telescope  will  describe  a  vertical  circle  when  the  telescope 
is  turned  on  the  horizontal  axis,  no  matter  to  what  part  of 
the  heavens  it  has  been  pointed. 

The  horizontal  and  vertical  axes  carry  each  a  pointer. 
These  pointers  move  over  the  graduated  circles,  and  mark 
how  far  each  axis  turns. 

To  find  the  azimuth  of  a  star,  the  instrument  is  turned 
on  its  vertical  axis  till  its  vertical  circle  is  brought  into  the 


14  ASTRONOMY. 

plane  of  the  meridian,  and  the  reading  of  the  horizontal 
circle  noted.  The  telescope  is  then  directed  to  the  star  by 
turning  it  on  both  its  vertical  and  horizontal  axes.  The 


Fig.  1 6. 

reading  of  the  horizontal  circle  is  again  noted.  The  differ- 
ence between  these  two  readings  of  the  horizontal  circle 
will  be  the  azimuth  of  the  star. 


ASTRONOMY.  15 

To  find  the  altitude  of  a  star,  the  reading  of  the  vertical 
circle  is  first  ascertained  when  the  telescope  is  pointed  hori- 
zontally, and  again  when  the  telescope  is  pointed  at  the  star. 
The  difference  between  these  two  readings  of  the  vertical 
circle  will  be  the  altitude  of  the  star. 

12.  The  Vernier.  —  To  enable  the  observer  to  read  the 
fractions  of  the  divisions  on  the  circles,  a  device  called  a 
vernier  is  often  employed.  It  consists  of  a  short,  graduated 
arc,  attached  to  the  end  of  an  arm  c  (Fig.  17),  which  is 
carried  by  the  axis,  and  turns  with  the  telescope.  This  arc 
is  of  the  length  of  nine  divisions  on  the  circle,  and  it  is 
divided  into  ten  equal  parts.  If  o  of  the  vernier  coincides 
with  any  division,  say  6,  of  the  circle,  i  of  the  vernier  will 
be  y1^  of  a  division  to 
the  left  of  7,  2  will  be 
•^  of  a  division  to  the 
left  of  8,  3  will  be  ^ 
of  a  division  to  the  left 
of  9,  etc.  Hence,  when 
i  coincides  with  7,  o 
will  be  at  6T\j ;  when  2 
coincides  with  8,  o  will 
be  at  6y40  ;  when  3  coincides  with  9,  o  will  be  at  6^,  etc. 

To  ascertain  the  reading  of  the  circle  by  means  of  the 
vernier,  we  first  notice  the  /ero  line.  If  it  exactly  coin- 
cides with  any  division  of  the  circle,  the  number  of  that 
division  will  be  the  reading  of  the  circle.  If  there  is  not 
an  exact  coincidence  of  the  zero  line  with  any  division  of 
the  circle,  we  run  the  eye  along  the  vernier,  and  note  which 
of  its  divisions  does  coincide  with  a  division  of  the  circle. 
The  reading  of  the  circle  will  then  be  the  number  of  the 
tirst  division  on  the  circle  behind  the  o  of  the  vernier,  and 
a  number  of  tenths  equal  to  the  number  of  the  division  of 
the  vernier,  which  coincides  with  a  division  of  the  circle. 
For  instance,  suppose  o  of  the  vernier  beyond  6  of  the 


i6 


ASTRONOMY. 


circle,  and   7   of  the   vernier  to   coincide   with    13   of   the 
circle.     The  reading  of  the  circle  will  then  be  6T7iy. 

13.  Hour  Circles.  —  Great   circles  perpendicular  to   the 
celestial  equator  are  called  hour  circles.     These  circles  all 

pass  through  the  poles  of 
the  heavens,  as  shown  in 
Fig.  1 8.  E  Q  is  the  celes- 
tial equator,  and  P  and 
P'  are  the  poles  of  the 
heavens. 

The  point  A  on  the 
equinoctial  (Fig.  19)  is 
called  the  vernal  equinox, 
or  the  first  point  of  Aries. 
The  hour  circle,  APP', 
which  passes  through  it,  is 
called  the  equinoctial  colure. 

14.  Declination  and  Right  Ascension. — ^^declination 
of  a  .heavenly  body  is  its   distance   north  or  south  of  the 
cejestial  equator.    The  polar 

distance  of  a  heavenly  body 
is  its  distance  from  the  nearer 
pole.  Declination  and  polar 
distance  are  measured  on 
hour  circles,  and  for  the  same 
heavenly  body  they  are  com- 
plements of  each  other. 

The  right  ascension  of  a 
heavenly  body  is  its  distance 
eastward  from  the  first  point 
of  Aries,  measured  from  the 
equinoctial  colure.  It  is  equal  to  the  arc  of  the  celestial 
equator  included  between  the  first  point  of  Aries  and  the 
hour  circle  which  passes  through  the  heavenly  body.  As 
right  ascension  is  measured  eastward  entirely  around  the 


ASTRONOMY.  l*J 

celestial  sphere,  it  may  have  any  value  from  o°  up  to  360°. 
Right  ascension  corresponds  to  longitude  on  the  earth,  and 
declination  to  latitude. 

15.  The  Meridian  Circle.  —  The  right  ascension  and 
declination  of  a  heavenly  body  are  ascertained  by  means  of 
an  instrument  called  the  meridian  circle,  or  transit  instru- 
ment. A  side-view  of  this  instrument  is  shown  in  Fig.  20. 


Fig.  20. 

It  consists  essentially  of  a  telescope  mounted  between  two 
piers,  so  as  to  turn  in  the  plane  of  the  meridian,  and  carry- 
ing a  graduated  circle.  The  readings  of  this  circle  are 
ascertained  by  means  of  fixed  microscopes,  under  which  it 
turns.  A  heavenly  body  can  be  observed  with  this  instru- 
ment, only  when  it  is  crossing  the  meridian.  For  this  reason 
it  is  often  called  the  transit  circle. 

To  find  the  declination  of  a  star  with  this  instrument,  we 


1 8  ASTRONOMY. 

first  ascertain  the  reading  of  the  circle  when  the  telescope 
is  pointed  to  the  pole,  and  then  the  reading  of  the  circle 
when  pointed  to  the  star  on  its  passage  across  the  meridian. 
The  difference  between  these  two  readings  will  be  the  polar 
distance  of  the  star,  and  the  complement  of  them  the  decli- 
nation of  the  star. 

To  ascertain  the  reading  of  the  circle  when  the  telescope 
is  pointed  to  the  pole,  we  must  select  one  of  the  circum- 
polar  stars  near  the  pole,  and  then  point  the  telescope  to 
it  when  it  crosses  the  meridian,  both  above  and  below  the 
pole,  and  note  the  reading  of  the  circle  in  each  case.  The 
mean  of  these  two  readings  will  be  the  reading  of  the  circle 
when  the  telescope  is  pointed  to  the  pole. 

1 6.  Astronomical  Clock.  —  An  astronomical  clock,  or  si- 
dereal clock  as  it  is  often  called,  is  a  clock  arranged  so  as 
to  mark  hours  from  i  to  24,  instead  of  from  i   to  12,  as  in 
the  case  of  an  ordinary  clock,  and  so  adjusted  as  to  mark 
o  when  the  vernal  equinox,  or  first  point  of  Aries,  is  on  the 
meridian. 

As  the  first  point  of  Aries  makes  a  complete  circuit  of 
the  heavens  in  twenty-four  hours,  it  must  move  at  the  rate 
of  15°  an  hour,  or  of  i°  in  four  minutes  :  hence,  when  the 
astronomical  clock  marks  i,  the  first  point  of  Aries  must  be 
15°  west  of  the  meridian,  and  when  it  marks  2,  30°  west  of 
the  meridian,  etc.  That  is  to  say,  by  observing  an  accurate 
astronomical  clock,  one  can  always  tell  how  far  the  meridian 
at  any  time  is  from  the  first  point  of  Aries. 

17.  How    to  find  Right  Ascension    with    the   Meridian 
Circle.  —  To  find  the  right  ascension  of  a  heavenly  body, 
we  have  merely  to  ascertain  the  exact  time,  by  the  astro- 
nomical clock,  at  which  the  body  crosses  the  meridian.     If 
a  star  crosses   the   meridian   at   i    hour  20   minutes  by  the 
astronomical   clock,  its  right  ascension   must   be  19°;   if  at 
20  hours,  its  right  ascension  must  be  300°. 

To  enable  the  observer  to  ascertain  with  great  exactness 


ASTRONOMY. 


the  time  at  which  a  star  crosses  the 
equidistant  and  parallel  spider- 
lines  are  stretched  across  the 
focus  of  the  telescope,  as  shown  in 
Fig.  2 1 .  The  observer  notes  the 
time  when  the  star  crosses  each 
spider-line  ;  and  the  mean  of  all 
of  these  times  will  be  the  time 
when  the  star  crosses  the  meridi- 
an. The  mean  of  several  obser- 
vations is  likely  to  be  more  nearly  j 
exact  than  any  single  observation. 
1 8.  The  Equatorial  Telescope. 


meridian,  a  number  of 


Fig.  22. 

fixed  in  any  declination,  and  then 


Fig.  21. 

—  The  equatorial  tele- 
scope is  mounted  on 
two  axes,  —  one  par- 
allel with  the  axis  of 
the  earth,  and  the 
other  at  right  angles 
to  this,  and  therefore 
parallel  with  the  plane 
,  of  the  earth's  equator. 
1  The  former  is  called 
the  polar  axis,  and 
the  latter  the  declina- 
tion axis.  Each  axis 
carries  a  graduated 
circle.  These  circles 
are  called  respective- 
ly the  hour  circle  and 
the  declination  circle. 
The  telescope  is  at- 
tached directly  to 
the  declination  axis. 
When, the  telescope  is 

turned  on  its  polar  axis. 


2O  ASTRONOMY. 

the  line  of  sight  will  describe  a  diurnal  circle  ;  so  that,  when 
the  tube  is  once  directed  to  a  star,  it  can  be  made  to  fol- 
low the  star  by  simply  turning  the  telescope  on  its  polar  axis. 

In  the  case  of  large  instruments  of  this  class,  the  polar 
axis  is  usually  turned  by  clock-work  at  the  rate  at  which  the 
heavens  rotate:  so  that,  when  the  telescope  has  once  been 
pointed  to  a  planet  or  other  heavenly  body,  it  will  continue  to 
follow  the  body  and  keep  it  steadily  in  the  field  of  view  without 
further  trouble  on  the  part  of  the  observer. 

The  great  Washington  Equatorial  is  shown  in  Fig.  22.  Its 
object-glass  is  26  inches  in  diameter,  and  its  focal  length  is 
32^  feet.  It  was  constructed  by  Alvan  Clark  &  Sons  of  Cam- 
bridge, Mass.  It  is  one  of  the  three  largest  refracting  tele- 
scopes at  present  in  use.  The  Newall  refractor  at  Gateshead. 
Eng.,  has  an  objective  25  inches  in  diameter,  and  a  focal  length 
of  29  feet.  The  great  refractor  at  Vienna  has  an  objective 
27  inches  in  diameter.  There  are  several  large  refractors  now 
in  process  of  construction. 

19.  The  Wire  Micrometer.  —  Large  arcs  in  the  heavens 
are  measured  by  means  of  the  graduated  circles  attached  to 

the  axes  of  the 
telescopes  ;  but 
small  arcs  within 
the  field  of  view- 
Fig.  23.  of  the  telescope 
are  measured  by  means  of  instruments  called  micrometers. 
mounted  in  the  focus  of  the  telescope.  One  of  the  most 
convenient  of  these  micrometers  is  that  known  as  the  wire 
micrometer,  and  shown  in  Fig.  23. 

The  frame  A  A  covers  two  slides,  C  and  D.  These  slides 
are  moved  by  the  screws  F  and  G.  The  wires  E  and  B 
are  stretched  across  the  ends  of  the  slides  so  as  to  be 
parallel  to  each  other.  On  turning  the  screws  F  and  G 
one  way,  these  wires  are  carried  apart ;  and  on  turning  them 
the  other  way  they  are  brought  together  again.  Sometimes 
two  parallel  wires,  x  and  jr,  shown  in  the  diagram  at  the 


ASTRONOMY.         .  21 

right,  are  stretched  across  the  frame  at  right  angles  to  the 
wires  E,  B.  We  may  call  the  wires  x  and  y  the  longitudi- 
nal wires  of  the  micrometer,  and  E  and  B  the  transverse 
wires.  Many  instruments  have  only  one  longitudinal  wire, 
which  is  stretched  across  the  middle  of  the  focus.  The 
longitudinal  wires  are  just  in  front  of  the  transverse  wires, 
but  do  not  touch  them. 

To  find  the  distance  between  any  two  points  in  the  field 
of  view  with  a  micrometer,  with  a  single  longitudinal  wire, 
turn  the  frame  till  the  longitudinal  wire  passes  through  the 
two  points  ;  then  set  the  wires  E  and  B  one  on  each 
point,  turn  one  of  the  screws,  known  as  the  micrometer 
screw,  till  the  two  wires  are  brought  together,  and  note  the 
number  of  times  the  screw  is  turned.  Having  previously 
ascertained  over  what  arc  one  turn  of  the  screw  will  move 
the  wire,  the  number  of  turns  will  enable  us  to  find  the 
length  of  the  arc  between  the  two  points. 

The  threads  of  the  micrometer  screw  are  cut  with  great 
accuracy  ;  and  the  screw  is  provided  with  a  large  head,  which 
is  divided  into  a  hundred  or  more  equal  parts. 

These  divisions,  by  means  of  a  fixed  pointer,  enable  us  to 
ascertain  what  fraction  of  a  turn  the  screw  has  made  over 
and  above  its  complete  revolutions. 

20.  Reflecting  Telescopes.  —  It  is  possible  to  construct 
mirrors  of  much  larger  size  than  lenses  :  hence  reflecting 
telescopes  have  an  advantage  over  refracting  telescopes  as 
regards  size  of  aperture  and  of  light-gathering  power.  They 
are,  however,  inferior  as  regards  definition  ;  and,  in  order 
to  prevent  flexure,  it  is  necessary  to  give  the  speculum,  or 
mirror,  a  massiveness  which  makes  the  telescope  unwieldy. 
It  is  also  necessary  frequently  to  repolish  the  speculum  ; 
and  this  is  an  operation  of  great  delicacy,  as  the  slightest 
change  in  the  form  of  the  surface  impairs  the  definition  of 
the  image.  These  defects  have  been  remedied,  to  a  certain 
extent,  by  the  introduction  of  silver-on-glass  mirrors  ;  that  is. 


22  ASTRONOMY.^ 

glass  mirrors  covered  in  front  with  a  thin  coating  of  silver. 
Glass  is  only  one-third  as  heavy  as  speculum-metal,  and  sil- 
ver is  much  superior  to  that  metal  in  reflecting  power ;  and 
when  the  stiver  becomes  tarnished,  it  can  be  removed  and 
renewed  without  danger  of  changing  the  form  of  the  glass. 

The  Herschelian  Reflector.  —  In  this  form  of  telescope  the 
mirror  is  slightly  tipped,  so  that  the  image,  instead  of  being 
formed  in  the  centre  of  the  tube,  is  formed  near  one  side 
of  it,  as  in  Fig.  24.  The  observer  can  then  view  it  with- 
out putting. his  head  inside  the  tube,  and  therefore  without 
cutting  off  any  material  portion  of  the  light.  In  observa- 
tion, he  must  stand  at  the  upper  or  outer  end  of  the  tube, 
and  look  into  it,  his  back  being  turned  towards  the  object. 
From  his  looking 
directly  into  the 
mirror,  it  is  also 
sometimes  called 
the  front -view 
telescope.  The 
great  disadvan- 
tage of  this  ar-  Fie-  24- 
rangement  is,  that  the  rays  cannot  be  brought  to  an  exact 
focus  when  they  are  thrown  so  far  to  one  side  of  the  axis. 
and  the  injury  to  the  definition  is  so  great  that  the  front- 
view  plan  is  now  entirely  abandoned. 

The  Newtonian  Reflector.  —  The  plan  proposed  by  Sir 
Isaac  Newton  was  to  place  a  small  plane  mirror  just  inside 
the  focus',  inclined  to  the  telescope  at  an  angle  of  45°,  so 
as  to  throw  the  rays  to  the  side  of  the  tube,  where  they 
come  to  a  focus,  and  form  the  image.  An  opening  is  made 
in  the  side  of  the  tube,  just  below  where  the  image  is 
formed ;  and  in  this  opening  the  eye-piece  is  inserted.  The 
small  mirror  cuts  off  some  of  the  light,  but  not  enough  to 
be  a  serious  defect.  An  improvement  which  lessens  this 
defect  has  been  made  by  Professor  Henry  Draper.  The 


ASTRONOMY.  23 

inclined  mirror  is  replaced  by  a  small  rectangular  prism 
(Fig.  25),  by  reflection  from  which  the  image  is  formed 
very  near  the  prism.  A  pair  of  lenses  are  then  inserted  in 
the  course  of  the  rays,  by  which  a  second  image  is  formed 
at  the  opening  in  the  side  of  the  tube  ;  and  this  second 
image  is  viewed  by  an  ordinary  eye-piece. 

The  Gregorian  Reflector.  —  This  is  a  form  proposed  by 


James  Gregory,  who  probably  preceded  Newton  as  an  in- 
ventor of  the  reflecting  telescope.  Behind  the  focus,  F 
(Fig.  26),  a  small  concave  mirror,  R,  is  placed,  by  which 
the  light  is  reflected  back  again  down  the  tube.  The  larger 
mirror,  M,  has  an  opening  through  its  centre  ;  and  the  small 
mirror,  R,  is  so  adjusted  as  to  form  a  second  image  of  the 
object  in  this  opening.  This  image  is  then  viewed  by  an 
eye-piece  which  is  screwed  into  the  opening. 


Fig.  26. 

The  Cassegrainian  Reflector.  —  In  principle  this  is  the 
same  with  the  Gregorian  ;  but  the  small  mirror,  R,  is  con- 
vex, and  is  placed  inside  the  focus,  F,  so  that  the  rays  are 
reflected  from  it  before  reaching  the  focus,  and  no  image 
is  formed  until  they  reach  the  opening  in  the  large  mirror. 
This  form  has  an  advantage  over  the  Gregorian,  in  that  the 


24  ASTRONOMY. 

telescope  may  be  made  shorter,  and  the  small  mirror  can 
be  more  easily  shaped  to  the  required  figure.  It  has,  there- 
fore, entirely  superseded  the  original  Gregorian  form. 


Optically  these  forms  of  telescope  are  inferior  to  the 
Newtonian  :  but  the  latter  is  subject  to  the  inconvenience, 
that  the  observer  must  be  stationed  at  the  upper  end  of 
the  telescope,  where  he  looks  into  an  eye-piece  screwed 
into  the  side  of  the  tube. 


ASTRONOMY.  25 

( )n  the  other  hand,  the  Cassegrainian  Telescope  is  pointed 
directly  at  the  object  to  be  viewed,  like  a  refractor ;  and  the 
observer  stands  at  the  lower  end,  and  looks  in  at  the  open- 
ing through  the  large  mirror.  This  is,  therefore,  the  most 
convenient  form  of  all  in  management. 


Fig.  28. 

The  largest  reflecting  telescope  yet  constructed  is  that  of 
Lord  Rosse,  at  Parsonstown,  Ireland..  Its  speculum  is  6  feet 
in  diameter,  and  its  focal  length  55  feet.  It  is  commonly  used 
as  a  Newtonian.  This  telescope  is  shown  in  Fig.  27. 

The  great  telescope  of  the  Melbourne  Observatory,  Austra- 
lia, is  a  Cassegranian  reflector.  Its  speculum  is  4  feet  in 


26 


ASTRONOMY. 


diameter,  and    its    focal   length    is    32    feet.     It   is    shown    in 
Fig.  28. 

The  great  reflector  of  the  Paris  Observatory  is  a  Newtonian 


Fig.  29. 

reflector.  Its  mirror  of  silvered  glass  is  4  feet  in  diameter, 
and  its  focal  length  is  23  feet.  This  telescope  is  shown  in 
Fig.  29. 

21.   The  Sun's  Motion  among  the  Stars.  —  If  we  notice 


ASTRONOMY. 


the  stars  at  the  same  hour  night  after  night,  we  shall  find 
that  the  constellations  are  steadily  advancing  towards  the 
west.  New  constellations  are  continually  appearing  in  the 
east,  and  old  ones  disappearing  in  the  west.  This  continual 
advancing  of  the  heavens  towards  the  west  is  due  to  the  fact 
that  the  sun's  place  among  the  stars  is  continually  moving 
towards  the  east.  The  sun  completes  the  circuit  of  the 
heavens  in  a  year,  and  is  therefore  moving  eastward  at  the 
rate  of  about  a  degree  a  day. 

This  motion  of  the  sun's  place  among  the  stars  is  due 
to  the  revolution  of 
the  earth  around  the 
sun,  and  not  to  any 
real  motion  of  the 
sun.  In  Fig.  30  sup- 
pose the  inner  circle 
to  represent  the  orbit 
of  the  earth  around 
the  sun,  and  the  outer 
circle  to  represent  the 
celestial  sphere.  When 
the  earth  is  at  E,  the 
sun's  place  on  the 
celestial  sphere  is  at  Fis-  3°-  . 

S'.  As  the  earth  moves  in  the  direction  E  F,  the  sun's 
place  on  the  celestial  sphere  must  move  in  the  direction 
.$"  T:  hence  the  revolution  of  the  earth  around  the  sun 
would  cause  the  sun's  place  among  the  stars  to  move  around 
the  heavens  in  the  same  direction  that  the  earth  is  moving 
around  the  sun. 

22.  The  Ecliptic.  —  The  circle  described  by  the  sun  in 
its  apparent  motion  around  the  heavens  is  called  the  ecliptic. 
The  plane  of  this  circle  passes  through  the  centre  of  the 
earth,  and  therefore  through  the  centre  of  the  celestial 
sphere  ;  the  earth  being  so  small,  compared  with  the  celes- 


28 


ASTRONOMY. 


tial  sphere,  that  it  practically  makes  no  difference  whether 
we  consider  a  point  on  its  surface,  or  one  at  its  centre,  as 
the  centre  of  the  celestial  sphere.  The  ecliptic  is,  therefore, 
a  great  circle. 

The  earth's  orbit  lies  in  the  plane  of  the  ecliptic  ;  but  it 
extends  only  an  inappreciable  distance  from  the  sun  towards 

the  celestial  sphere. 

23.  TJi c  Obliquity  of  the 
Ecliptic.  —  The  ecliptic  is  in- 
clined to  the  celestial  equator 
by  an  angle  of  about  23!°. 
This  inclination  is  called  the 
obliquity  of  the  ecliptic.  The 
obliquity  of  the  ecliptic  is  due 

to  the  deviation  of  the  earth's  axis  from  a  perpendicular  to 
the  plane  of  its  orbit.  The  axis  of  a  rotating  body  tends 
to  maintain  the  same  direction  ;  and,  as  the  earth  revolves 
around  the  sun,  its  axis  points  all  the  time  in  nearly  the 
same  direction.  The  earth's  axis  deviates  about  23^°  from 
the  perpendicular  to  its  orbit ;  and,  as  the  earth's  equator  is 
at  right  angles  to  its  axis,  it 
will  deviate  about  23!-°  from 
the  plane  of  the  ecliptic. 
The  celestial  equator  has  the 
same  direction  as  the  terres- 
trial equator,  since  the  axis 
of  the  heavens  has  the  same 
direction  as  the  axis  of  the 
earth.  **  32> 

Suppose  the  globe  at  the  centre  of  the  tub  (Fig.  31)  to 
represent  the  sun,  and  the  smaller  globes  to  represent  the 
earth  in  various  positions  in  its  orbit.  The  surface  of  the 
water  will  then  represent  the  plane  of  the  ecliptic,  and 
the  rod  projecting  from  the  top  of  the  earth  will  represent 
the  earth's  axis,  which  is  seen  to  point  all  the  time  in  the 


ASTRONOMY. 


29 


same  direction,  or  to  lean  the  same  way.     The  leaning  of 

the  axis  from  the  perpendicular  to  the  surface  of  the  water 

would  cause  the  earth's  equator  to  be  inclined    the  same 

amount  to  the  surface  of  the  water,  half  of  the  equator  being 

above,  and  half  of  it  below, 

the  surface.     Were  the  axis 

of    the    earth    perpendicular 

to  the  surface  of  the  water, 

the    earth's    equator    would 

coincide  with  the  surface,  as 

is  evident  from  Fig.  32. 

24.  The    Equinoxes  and 
Solstices.  —  The  ecliptic  and 
celestial  equator,  being  great 
circles,    bisect    each    other. 
Half  of  the  ecliptic  is  north, 

and  half  of  it  is  south,  of  the  equator.  The  points  at  which 
the  two  circles  cross  are  called  the  equinoxes.  The  one  at 
which  the  sun  crosses  the  equator  from  south  to  north  is 
called  the  vernal  equinox,  and  the  one  at  which  it  crosses 

from  north  to  south  the 
autumnal  equinox.  The 
points  on  the  ecliptic  mid- 
way between  the  equinoxes 
are  called  the  solstices.  The 
one  north  of  the  equator  is 
called  the  summer  solstice, 
and  the  one  south  of  the 
equator  the  winter  solstice. 
In  Fig.  33,  E  Q  is  the  celes- 
tial equator,  EC  Er c'  the 
ecliptic,  V  the  vernal  equinox,  A  the  autumnal  equinox,  E  c 
the  winter  solstice,  and  E' c'  the  summer  solstice. 

25.  The  Inclination  of  the  Ecliptic    to   the  Horizon. — 
Since  the  celestial  equator  is  perpendicular  to  the  axis  of 


Fig.   34- 


3O  ASTRONOMY. 

the  heavens,  it  makes  the  same  angle  with  it  on  every  side  : 
hence,  at  any  place,  the  equator  makes  always  the  same 
angle  with  the  horizon,  whatever  part  of  it  is  above  the  hori- 
zon. But,  as  the  ecliptic  is  oblique  to  the  equator,  it  makes 
different  angles  with  the  celestial  axis  on  different  sides  ; 
and  hence,  at  any  place,  the  angle  which  the  ecliptic  makes 
with  the  horizon  varies  according  to  the  part  which  is  above 
the  horizon.  The  two  extreme  angles  for  a  place  more  than 
23!°  north  of  the  equator  are  shown  in  Figs.  34  and  35. 

The  least  angle  is  formed  when  the  vernal  equinox  is  on 
the  eastern  horizon,  the  autumnal  on  the  western  horizon, 
and  the  winter  solstice  on  the  meridian,  as  in  Fig.  34.  The 

angle  which  the  ecliptic  then 
makes  with  the  horizon  is 
equal  to  the  elevation  of  the 
equinoctial  minus  23^°.  In 
the  latitude  of  New  York  this 
angle  =  49°  -  23}°  -  25^°. 
The  greatest  angle  is 
formed  when  the  autumnal 
equinox  is  on  the  eastern 
horizon,  the  vernal  on  the 
western  horizon,  and  the 

summer  solstice  is  on  the  meridian  (Fig.  35).  The  angle 
between  the  ecliptic  and  the  horizon  is  then  equal  to  the 
elevation  of  the  equinoctial  plus  23 -J°.  In  the  latitude  of 
New  York  this  angle  —  49°  -f  23 J°  =  72^°. 

Of  course  the  equinoxes,  the  solstices,  and  all  other  points 
on  the  ecliptic,  describe  diurnal  circles,  like  every  other 
point  in  the  heavens  :"  hence,  in  our  latitude,  these  points 
rise  and  set  every  day. 

26.  Celestial  Latitude  and  Longitude.  —  Celestial  lati- 
tude is  distance  measured  north  or  south  from  the  ecliptic  ; 
and  celestial  longitude  is  distance  measured  on  the  ecliptic 
eastward  from  the  vernal  equinox,  or  the  first  point  of 


ASTRONOMY.  31 

Aries.  Great  circles  perpendicular  to  the  ecliptic  are  called 
celestial  meridians.  These  circles  all  pass  through  the 
poles  of  the  ecliptic,  which  are  some  23^°  from  the  poles  of 
the  equinoctial.  The  latitude  of  a  heavenly  body  is  meas- 
ured by  the  arc  of  a  celestial  meridian  included  between 
the  body  and  the  ecliptic.  The  longitude  of  a  heavenly 
body  is  measured  by  the  arc  of  the  ecliptic  included  be- 
tween the  first  point  of  Aries  and  the  meridian  which  passes 
through  the  body.  There  are,  of  course,  always  two  arcs 
included  between  the  first  point  of  Aries  and  the  meridian, 
—  one  on  the  east,  and  the  other  on  the  west,  of  the  first 
point  of  Aries.  The  one  on  the  east  is  always  taken  as 
the  measure  of  the  longitude. 
27.  The  Precession  of  the 
Equinoxes.  —  The  equinoc- 
tial points  have  a  slow  west- 
ward motion  along  the  eclip- 
tic. This  motion  is  at  the 
rate  of  about  50"  a  year,  and 
would  cause  the  equinoxes 
to  make  a  complete  circuit 
of  the  heavens  in  a  period 
of  about  twenty-six  thousand 
years.  It  is  called  the  precession  of  the  equinoxes.  This 
westward  motion  of  the  equinoxes  is  due  to  the  fact  that 
the  axis  of  the  earth  has  a  slow  gyratory  motion,  like  the 
handle  of  a  spinning-top  which  has  begun  to  wabble  a  little. 
This  gyratory  motion  causes  the  axis  of  the  heavens  to 
describe  a  cone  in  about  twenty-six  thousand  years,  and  the 
pole  of  the  heavens  to  describe  a  circle  about  the  pole  of 
the  ecliptic  in  the  same  time.  The  radius  of  this  circle 


Fig.  36. 


28.  Illustration  of  Precession.  —  The  precession  of  the 
equinoxes  may  be  illustrated  by  means  of  the  apparatus  shown 
in  Fig.  36.  The  horizontal  and  stationary  ring  E  C  represents 


32  ASTRONOMY. 

the  ecliptic ;  the  oblique  ring  E'  Q  represents  the  equator : 
V  and  A  represent  the  equinoctial  point,  and  E  and  C  the 
solstitial  points ;  B  represents  the  pole  of  the  ecliptic,  P  the 
pole  of  the  equator,  and  P  O  the  celestial  axis.  The  ring  E'  Q 
is  supported  on  a  pivot  at  Oj  and  the  rod  B  P,  which  connects 
B  and  P,  is  jointed  at  each  end  so  as  to  admit  of  the  move- 
ment of  P  and  B. 

On  carrying  P  around  B,  we  shall  see  that  E'  Q  will  always 
preserve  the  same  obliquity  to  EC,  and  that  the  points  Fand  A 
will  move  around  the  circle  E  C.  The  same  will  also  be  true 
of  the  points  E  and  C. 

29.  Effects  of  Precession.  —  One  effect  of  precession,  as 
has  already  been  stated,  is  the  revolution  of  the  pole  of  the 
heavens  around  the  pole  of  the  ecliptic  in  a  period  of  about 
twenty-six  thousand  years.  The  circle  described  by  the 
pole  of  the  heavens,  and  the  position  of  the  pole  at  various 
dates,  are  shown  in  Fig.  37,  where  o  indicates  the  position 
of  the  pole  at  the  birth  of  Christ.  The  numbers  round 
the  circle  to  the  left  of  o  are  dates  A.D.,  and  those  to  the 
right  of  o  are  dates  B.C.  It  will  be  seen  that  the  star  at 
the  end  of  the  Little  Bear's  tail,  which  is  now  near  the 
north  pole,  will  be  exactly  at  the  pole  about  the  year  2000. 
It  will  then  recede  farther  and  farther  from  the  pole  till  the 
year  15000  A.D.,  when  it  will  be  about  forty-seven  degrees 
away  from  the  pole.  It  will  be  noticed  that  one  of  the  stars 
of  the  Dragon  was  the  pole  star  about  2800  years  B.C. 
There  are  .reasons  to  suppose  that  this  was  about  the  time 
of  the  building  of  the  Great  Pyramid. 

A  second  effect  of  precession  is  the  shifting  of  the  signs 
along  the  zodiac.  The  zodiac  is  a  belt  of  the  heavens 
along  the  ecliptic,  extending  eight  degrees  from  it  on  each 
side.  This  belt  is  occupied  by  twelve  constellations,  known 
as  the  zodiacal  constellations.  They  are  Aries,  Taurus, 
Gemini,  Cancer,  Leo,  Virgo.  Libra,  Scorpio,  Sagittarius. 
Capricornus,  Aquarius,  and  Pisces.  The  zodiac  is  also 


ASTRONOMY.  33 

divided  into  twelve  equal  parts  of  thirty  degrees  each,  called 
signs.  These  signs  have  the  same  names  as  the  twelve 
zodiacal  constellations,  and  when  they  were  first  named, 
each  sign  occupied  the  same  part  of  the  zodiac  as  the  cor- 
responding constellation ;  that  is  to  say,  the  sign  Aries  was 
in  the  constellation  Aries,  and  the  sign  Taurus  in  the  con- 


•f  *       *  ^:-'\ 

g     "  *y     V*'^ 

*r--C  «•  it  r* 

^:   ^\    rV*»   $VJ 


MOy 


••' J  a.         /    .  / 

ft  i>^,/f  L 

y  V,-  irr 


•«*\          u    >,i  / 

^CX  \  If  cad  of;    "^  / 

Tr^     **tw.     /" 


, 
lyre 


Fig.  37- 

stellation  Taurus,  etc.  Now  the  signs  are  always  reckoned 
as  beginning  at  the  vernal  equinox,  which  is  continually 
shifting  along  the  ecliptic  ;  so  that  the  signs  are  continually 
moving  along  the  zodiac,  while  the  constellations  remain 
stationary  :  hence  it  has  come  about  that  the  first  point  of 
Aries  (the  sign)  is  no  longer  in  the  constellation  Aries,  but 
in  Pisces. 


34  ASTRONOMY. 

Fig.  38  shows  the  position  of  the  vernal  equinox  2170 
B.C.  It  was  then  in  Taurus,  just  south  of  the  Pleiades. 
It  has  since  moved  from  Taurus,  through  Aries,  and  into 
Pisces,  as  shown  in  Fig.  39. 


Fig.  38. 

Since  celestial  longitude  and  right  ascension  are  both 
measured  from  the  first  point  of  Aries,  the  longitude  and 
right  ascension  of  the  stars  are  slowly  changing  from  year 


Fig.  39- 

to  year.     It  will  be  seen,  from  Figs.   38  and  39,  that  the 
declination  is  also  slowly  changing. 

30.  Nutation.  —  The  gyratory  motion  of  the  earth's  axis 
is  not  perfectly  regular  and  uniform.     The  earth's  axis  has 


ASTRONOMY.  35 

a  slight  tremulous  motion,  oscillating  to  and  fro  through  a 
short  distance  once  in  about  nineteen  years.  This  tremu- 
lous motion  of  the  axis  causes  the  pole  of  the  heavens  to 
describe  an  undulating  curve,  as  shown  in  Fig.  40,  and 
gives  a  slight  uneven  ness  to  the  motion  of  the  equinoxes 
along  the  ecliptic.  This  nodding  motion  of  the  axis  is 
railed  nutation. 

31.  Refraction.  —  When  a  ray  of  light 
from  one  of  the  heavenly  bodies  enters  the 
earth's  atmosphere  obliquely,  it  will  be  bent 
towards  a  perpendicular  to  the  surface  of 
the  atmosphere,  since  it  will  be  entering  a 
denser  medium.  As  the  ray  traverses  the  Flg>  4C> 

atmosphere,  it  will  be  continually  passing  into  denser  and 
denser  layers,  and  will  therefore  be  bent  more  and  more 
towards  the  perpendicular.  This  bending  of  the  ray  is 
shown  in  Fig.  41.  A  ray  which  started  from  A  would  enter 
the  eye  at  C.  as  if  it  came  from  /.•  hence  a  star  at  A  would 
appear  to  be  at  /. 

Atmospheric  refraction  displaces  all  the  heavenly  bodies 
from  the  horizon  towards  the 
/enith.  This  is  evident  from 
Fig.  42.  OD  is  the  horizon, 
and  Z  the  zenith,  of  an  observer 
at  O.  Refraction  would  make 
a  star  at  Q  appear  at  P:  in 
other  words,  it  would  displace 
it  towards  the  zenith.  A  star  in 
the  zenith  is  not  displaced  by 
refraction,  since  the  rays  which  reach  the  eye  from  it  traverse 
the  atmosphere  vertically.  The  farther  a  star  is  from  the 
zenith,  the  more  it  is  displaced  by  refraction,  since  the 
greater  is  the  obliquity  with  which  the  rays  from  it  enter 
the  atmosphere. 

At   the   horizon   the   displacement  by  refraction  is  about 


36  ASTRONOMY. 

half  a  degree  ;  but  it  varies  considerably  with  the  state  of 
the    atmosphere.     Refraction    causes    a   heavenly    body   to 


Fig.  42- 


appear  above  the  horizon  longer  than  it  really  is  above  it. 
since  it  makes  it  appear  to  be  on  the  horizon  when'  it  is 
really  half  a  degree  below  it. 


Fig.  43- 

The    increase    of    refraction    towards    the    horizon    often 
makes   the   sun,  when   near   the   horizon,  appear  distorted, 


ASTRONOMY. 


37 


the  lower  limb  of  the  sun  being  raised  more  than  the  upper 
limb.  This  distortion  is  shown  in  Fig.  43.  The  vertical 
diameter  of  the  sun  appears  to  be  considerably  less  than 
the  horizontal  diameter. 

32.  Parallax.  —  Parallax  is  the  displacement  of  an 
object  caused  by  a  change  in  the  point  of  view  from  which 
it  is  seen.  Thus  in  Fig.  44,  the  top  of  the  tower  S  would 
be  seen  projected  against  the  sky  at  a  by  an  observer  at  A, 
and  at  b  by  an  observer  at  B.  In  passing  from  A  to  B, 
the  top  of  the  tower  is  displaced  from  a  to  £,  or  by  the 


angle  a  Sb.     This  angle  is  called  the  parallax  of  S,  as  seen 
from  B  instead  of  A. 

The  geocentric  parallax  of  a  heavenly  body  is  its  dis- 
placement caused  by  its  being  seen  from  the  surface  of  the 
earth,  instead  of  from  the  centre  of  the  earth.  In  Fig.  45. 
R  is  the  centre  of  the  earth,  and  O  the  point  of  observation 
on  the  surface  of  the  earth.  Stars  at  S,  S',  and  S",  would. 
Trom  the  centre  of  the  earth,  appear  at  Q,  Q',  and  Q" ; 
while  from  the  point  O  on  the  surface  of  the  earth,  these 
same  stars  would  appear  at  P,  Pf,  and  /*",  being  displaced 


ASTRONOMY. 


from  their  position,  as  seen  from  the  centre  of  the  earth,  by 
the  angles  QSP,  Q'S'P',  and  Q" S" P" .  It  will  be  seen 
that  parallax  displaces  a  body  from  the  zenith  towards  the 
horizon,  and  that  the  parallax  of  a  body  is  greatest  when  it 
is  on  the  horizon.  The  parallax  of  a  heavenly  body  when 
on  the  horizon  is  called  its  horizontal  parallax.  A  body 
in  the  zenith  is  not  displaced  by  parallax,  since  it  would 
be  seen  in  the  same  direction  from  both  the  centre  and 

the  surface  of  the 
earth. 

The  parallax  of 
a  body  at  S"'  is 
Q'"S'"P,  which  is 
seen  to  be  greater 
than  QSP;  that 
is  to  say,  the  par- 
•B  allax  of  a  heaven- 
ly body  increases 
with  its  nearness 
to  the  earth.  The 
distance  and  parallax  of  a  body  are  so  related,  that,  either 
being  known,  the  other  may  be  computed. 

33.  Aberration.  —  Aberration  is  a  slight  displacement  of  a 
star,  owing  to  an  apparent  change  in  the  direction  of  the  rays 
of  light  which  proceed  from  it.  caused  by  the  motion  of  the 
earth  in  its  orbit.  If  we  walk  rapidly  in  any  direction  in  the 
rain,  when  the  drops  are  falling  vertically,  they  will  appear  to 
come  into  our  faces  from  the  direction  in  which  we  are  walking. 
Our  own  motion  has  apparently  changed  the  direction  in  which 
the  drops  are  falling. 

In  Fig.  46  let  A  be  a  gun  of  a  battery,  from  which  a  shot 
is  fired  at  a  ship,  D E,  that  is  passing.  Let  ABC  be  the 
course  of  the  shot.  The  shot  enters  the  ship's  side  at  J3,  and 
passes  out  at  the  other  side  at  C;  but  in  the  mean  time  the 
ship  has  moved  from  E  to  e.  and  the  part  B.  where  the  shot 
entered,  has  been  carried  to  b.  If  a  person  on  board  the  ship 


ASTRONOMY.  39 

could  see  the  ball  as  it  crossed  the  ship,  he  would  see  it  cross 
in  the  diagonal  line  b  C;  and  he  would  at  once  say  that  the 
cannon  was  in  the  direction  of  Cb.  If  the  ship  were  moving 
in  the  opposite  direction,  he  would 
say  that  the  cannon  was  just  as  far 
the  other  side  of  its  true  position. 

Xow,  we  see  a  star  in  the  direction 
in  which  the  light  coming  from  the 
star  appears  to  be  moving.  When 
we  examine  a  star  with  a  telescope, 
we  are  in  the  same  condition  as  the 
person  who  on  shipboard  saw  the 
cannon-ball  cross  the  ship.  The  tele- 
scope is  carried  along  by  the  earth 

at  the  rate  of  eighteen   miles  a  second :    hence  the  light  will 
appear  to  pass  through  the  tube  in  a  slightly  different  direction 
from  that  in  which  it  is  really  moving:  just  as  the  cannon-ball 
appears  to  pass  through  the  ship  in  a  different  direc- 
tion from  that  in  which  it  is  really  moving.     Thus  in 
Fig.  47,  a  ray  of  light  coming  in  the  direction  SOT 
would  appear  to  traverse  the  tube  O  T  of  a  telescope, 
moving  in  the  direction    of    the  arrow,  as  if  it  were 
coming  in  the  direction  S'  O. 

As  light  moves  with  enormous  velocity,  it  passes 
through  the  tube  so  quickly,  that  it  is  apparently 
changed  from  its  true  direction  only  by  a  very  slight 
angle:  but  it  is  sufficient  to  displace  the  star.  This 
apparent  change  in  the  direction  of  light  caused  by 
the  motion  of  the  earth  is  called  aberration  of  light. 

34.   The  Planets.  —  On  watching  the  stars  atten- 
tively night  after  night,  it  will  be  found,  that  while 
the  majority  of  them  appear  fixed  on  the  surface 
of  the  celestial  sphere,  so  as  to  maintain  their  rela- 
tive positions,  there  are  a  few  that  wander  about 
among  the  stars,  alternately  advancing  towards  the     Flg>  47' 
east,  halting,  and  retrograding  towards  the  west.     These  wan- 
dering stars  are  called  planets. 

Their  motions  appear  quite  irregular ;  but,  on  the  whole. 


4O  ASTRONOMY. 

their  eastward  motion  is  in  excess  of  their  westward,  and  in 
a  longer  or  shorter  time  they  all  complete  the  circuit  of  the 
heavens.  In  almost  every  instance,  their  paths  are  found  to 
lie  wholly  in  the  belt  of  the  zodiac. 


Fig.  48. 


Fig.  48  shows  a  portion  of  the  apparent  path  of  one  of 
the  planets. 


II. 

THE   SOLAR  SYSTEM. 


I.     THEORY    OF   THE    SOLAR    SYSTEM. 

35.  Members  of  the  Solar  System. — The    solar   system 
is  composed  of  the  sun,  planets,  moons,  comets,  and  meteors. 
Five  planets,  besides  the  earth,  are  readily  distinguished  by 
the  naked  eye,  and  were  known  to  the  ancients :  these  are 
Mercury,  Venus,  Mars,  Jupiter,  and  Saturn.     These,  with 
the  sun  and  moon,  made  up  the  seven  planets  of  the  ancients, 
from  which  the  seven  days  of  the  week  were  named. 

THE  PTOLEMAIC  SYSTEM. 

36.  The  Crystalline  Spheres.  —  We  have  seen  that  all  the 
heavenly  bodies  appear  to  be  situated  on  the  surface  of  the 
celestial  sphere.     The  ancients  assumed  that  the  stars  were 
really  fixed  on  the  surface  of  a  crystalline  sphere,  and  that 
they  were  carried  around  the  earth  daily  by  the  rotation  of 
this  sphere.     They  had,  however,  learned  to  distinguish  the 
planets  from  the  stars,  and  they  had  come  to  the  conclusion 
that  some  of  the  planets  were  nearer  the  earth  than  others, 
and  that  all  of  them  were  nearer  the  earth  than  the  stars 
are.     This  led  them  to  imagine  that  the  heavens  were  com- 
posed of  a  number  of  crystalline  spheres,  one  above  another, 
each  carrying  one  of  the  planets,  and  all  revolving  around 
the  earth   from  east   to  west,  but   at   different   rates.     This 
structure  of  the  heavens  is  shown  in  section  in  Fig.  49. 

41 


42  ASTRONOMY. 

37.  Cycles  and  Epicycles. — The  ancients  had  also  noticed 
that,  while  all  the  planets  move  around  the  heavens  from 
west  to  east,  their  motion  is  not  one  of  uniform  advance- 
ment. Mercury  and  Venus  appear  to  oscillate  to  and  fro 
across  the  sun,  while  Jupiter  and  Saturn  appear  to  oscillate 
to  and  fro  across  a  centre  which  is  moving  around  the 
earth,  so  as  to  describe  a  series  of  loops,  as  shown  in 
Fig.  50. 


Fig.  49. 

The  ancients  assumed  that  the  planets  moved  in  exact 
circles,  and,  in  fact,  that  all  motion  in  the  heavens  was 
circular,  the  circle  being  the  simplest  and  most  perfect 
curve.  To  account  for  the  loops  described  by  the  planets, 
they  imagined  that  each  planet  revolved  in  a  circle  around  a 
centre,  which,  in  turn,  revolved  in  a  circle  around  the  earth. 
The  circle  described  by  this  centre  around  the  earth  they 
called  the  cycle,  and  the  circle  described  by  the  planet 
around  this  centre  they  called  the  epicycle. 


ASTRONOMY. 


43 


38.  The  Eccentric.  —  The  ancients  assumed  that  the 
planets  moved  at  a  uniform  rate  in  describing  the  epicycle, 
and  also  the  centre  in  describing  the  cycle.  They  had, 
however,  discovered  that  the  planets  advance  eastward  more 
rapidly  in  some  parts  of  their  orbits  than  in  others.  To 


account  for  this  they  assumed  that  the  cycles  described  by 
the  centre,  around  which  the  planets  revolved,  were  eccen- 
tric;  that  is  to  say,  that  the  earth  was  not  at  the  centre 
of  the  cycle,  but  some  distance  away  from  it,  as  shown 
in  Fig.  51.  E  is  the  position  of  the  earth,  and  C  is  the 


44 


ASTRONOMY. 


centre  of  the  cycle.  The  lines  from  E  are  drawn  so  as  to 
intercept  equal  arcs  of  the  cycle.  It  will  be  seen  at  once 
that  the  angle  between  any  pair  of  lines  is  greatest  at  P,  and 

least  at  A ;  so  that,  were  a 
planet  moving  at  the  same  rate 
at  P  and  A,  it  would  seem  to 
be  moving  much  faster  at  P. 
The  point  P  of  the  planet's 
cycle  was  called  its  perigee,  and 
the  point  A  its  apogee. 

As  the   apparent  motion   of 
the   planets   became   more   ac- 
curately  known,   it  was    found 
necessary  to   make   the  system 
si-  of  cycles,  epicycles,  and  eccen- 

trics exceedingly  complicated  to  represent  that  motion. 

THE  COPERNICAN  SYSTEM. 

39.  Copernicus.  —  Copernicus    simplified    the    Ptolemaic 
system  greatly  by  assuming  that  the  earth  and  all  the  planets 
revolved  about  the  sun  as  a  centre.     He,  however,  still  main- 
tained that  all  motion  in  the  heavens  was  circular,  and  hence 
he  could  not  rid  his  system  entirely  of  cycles  and  epicycles. 

TYCHO  BRAKE'S  SYSTEM. 

40.  Tycho  Brake. — Tycho  Brahe  was  the  greatest  of  the 
early  astronomical  observers.     He,  however,  rejected  the  sys- 
tem of  Copernicus,  and  adopted  one  of  his  own,  which  was 
much  more  complicated.     He  held  that  all  the  planets  but 
the  earth  revolved  around  the  sun.  while  the  sun  and  moon 
revolved  around  the  earth.     This  system  is  shown  in  Fig.  52. 

KEPLER'S  SYSTEM. 

41.  Kepler.  —  While  Tycho  Brahe  devoted  his  life  to  the 
observation   of  the   planets.  Kepler  gave   his  to   the   study 


ASTRONOMY. 


45 


of  Tycho's  observations,  for  the  purpose  of  discovering  the 
true  laws  of  planetary  motion.  He  banished  the  compli- 
cated system  of  cycles,  epicycles,  and  eccentrics  forever  from 
the  heavens,  and  discovered  the  three  laws  of  planetary 
motion  which  have  rendered  his  name  immortal. 

42.  The  Ellipse.  —  An  ellipse  is  a  closed  curve  which  has 
two  points  within  it,  the  sum  of  whose  distances  from  every 
point  on  the  curve  is  the  same.  These  two  points  are  called 
of  the  ellipse. 


Fig.  52. 

One  method  of«  describing  an  ellipse  is  shown  in  Fig.  53. 
Two  tacks,  F  and  F',  are  stuck  into  a  piece  of  paper,  and 
to  these  are  fastened  the  two  ends  of  a  string  which  is  longer 
than  the  distance  between  the  tacks.  A  pencil  is  then 
placed  against  the  string,  and  carried  around,  as  shown  in 
the  figure.  The  curve  described  by  the  pencil  is  an  ellipse. 
The  two  points  F  and  F'  are  the  foci  of  the  ellipse  :  the 
sum  of  the  distances  of  these  two  points  from  every  point 
on  the  curve  is  equal  to  the  length  of  the  string.  When 
half  of  the  ellipse  has  been  described,  the  pencil  must  be 


46  ASTRONOMY. 

held  against  the  other  side  of  the  string  in  the  same  way, 
and  carried  around  as  before. 

The  point  O,  half  way  between  F  and  F' ,  is  called  the 


t'ig-   53- 

centre  of  the  ellipse  ;  A  A'  is  the  major  axis  of  the  ellipse, 
and  CD  is  the  minor  axis. 

43.  The  Eccentricity  of  the  Ellipse.  —  The  ratio  of  the 
distance   between  the   two  foci   to    the    major  axis  of  the 
ellipse  is  called  the  eccentricity  of  the  ellipse.     The  greater 

the  distance  between  the  two 
foci,  compared  with  the  major 
axis  of  the  ellipse,  the  greater  is 
the  eccentricity  of  the  ellipse ; 
and  the  less  the  distance  be- 
tween the  foci,  compared  with 
the  length  of  the  major  axis, 
the  less  the  eccentricity  of  the 
ellipse.  The  ellipse  of  Fig.  54 
has  an  eccentricity  of  J-.  This 
Flg-  54-  ellipse  scarcely  differs  in  appear- 

ance from  a  circle.     The  ellipse  of  Fig.  55  has  an  eccen- 
tricity of  \,  and  that  of  Fig.  56  an  eccentricity  of  J. 

44.  Kepler's  First  Law.  —  Kepler  first  discovered  that 
all  the  planets  move  from  west  to  east  in  ellipses  which  have 


ASTRONOMY. 


47 


Fig.  55- 


the  sun  as  a  common  focus.  This  law  of  planetary  motion 
is  known  as  Kepler's  First  Law.  The  planets  appear  to 
describe  loops,  because  we  view  them  from  a  moving  point. 

The  ellipses  described  by  the  planets  differ  in  eccentricity  ; 
and,  though  they  all  have  one  focus  at  the  sun,  their  major 
axes  have  different  directions.  The  eccentricity  of  the  plan- 
etary orbits  is  comparatively 
small.  The  ellipse  of  Fig.  54 
has  seven  times  the  eccentricity 
of  the  earth's  orbit,  and  twice 
that  of  the  orbit  of  any  of  the 
larger  planets  except  Mercury  ; 
and  its  eccentricity  is  more 
than  half  of  that  of  the  orbit 
of  Mercury.  Owing  to  their 
small  eccentricity,  the  orbits  of 
the  planets  are  usually  represented  by  circles  in  astronomi- 
cal diagrams. 

45.  Kepler's  Second  Law.  —  Kepler  next  discovered  that 
a  planet's  rate  of  motion  in  the  various  parts  of  its  orbit 
is  such  that  a  line  drawn  from  the  planet  to  the  sun  would 
always  sweep  over  equal  areas  in  equal  times.  Thus,  in 
Fig.  57,  suppose  the  planet  would  move  from  Pio  Pl  in  the 

same  time  that  it  would  move 
from  P2  to  PI,  or  from  P*  to 
P5 ;  then  the  dark  spaces,  which 
would  be  swept  over  by  a  line 
joining  the  sun  and  the  planet, 
in  these  equal  times,  would  all 
be  equal. 

A  line  drawn  from  the  sun  to  a  planet  is  called  the 
radius  vector  of  the  planet.  The  radius  vector  of  a  planet 
is  shortest  when  the  planet  is  nearest  the  sun,  or  at  perihe- 
lion, and  longest  when  the  planet  is  farthest  from  the  sun, 
or  at  aphelion  :  hence,  in  order  to  have  the  areas  equal,  it 


Fig.  56- 


48  ASTRONOMY. 

follows  that  a  planet  must  move  fastest  when  at  perihelion, 
and  slowest  at  aphelion. 

Kepler's  Second  Law  of  planetary  motion  is  usually 
stated  as  follows  :  The  radius  vector  of  a  planet  describes 
equal  areas  in  equal  times  in  every  part  of  the  planet's 
orbit. 

46.  Kepler's  Third  Law.  —  Kepler  finally  discovered  that 

the  -periodic    times 

,-_— . ^^^^^p  0^"  tne  planets  bear 

P/xX""'  ^^^^^^^"~\.  the    following    rela- 

^^L  ^^^^^  N.  tion    to  the  distan- 

Jl  ^^S^^^  \       ces   °^  ^e    planets 

^P^^^^V^Mj^^^^M1"  fr°m  the   s'm  :    The 

^^^jpt .  squares  of  the  pen- 

^N^^  /        odic    times    of    the 

^•^^^^  ^^X  planets  are  to  each 

"""  other  as    the    cubes 

of  their  "mean  dis- 
tances from  the  sun.  This  is  known  as  Kepler ' s  Third  Law 
of  planetary  motion.  By  periodic  time  is  meant  the  time  it 
takes  a  planet  to  revolve  around  the  sun. 

These  three  laws  of  Kepler's  are  the  foundation  of  mod- 
ern physical  astronomy. 

THE  NEWTONIAN  SYSTEM. 

47.  Newton's  Discovery.  —  Newton  followed  Kepler,  and 
by  means  of  his  three  laws  of  planetary  motion  made  his 
own  immortal  discovery  of  the  law  of  gravitation.     This 
law  is  as  follows  :  Every  portion  of  matter  in   the  universe 
attracts  every  other  portion  with  a  force  varying  directly  as 
the  product  of  the  masses  acted  upon,  and  inversely  as  the 
square  of  the  distances  between  them. 

48.  The    Conic   Sections.  —  The   conic   sections   are    the 
figures  formed  by  the  various  plane  sections  of  a  right  cone. 
There  are  four  classes  of  figures  formed  by  these   sections, 


ASTRONOMY. 


49 


according   to    the    angle  which   the    plane   of  the    section 
makes  with  the  axis  of  the  cone. 

OPQ,  Fig.  58,  is  a  right  cone,  and  ON  is  its  axis. 
Any  section,  A  B,  of  this  cone,  whose  plane  is  perpendicular 
to  the  axis  of  the  cone,  is  a  circle. 

Any  section,  CD,  of  this  cone,  whose  plane  is  oblique 
to  the  axis,  but  forms  with  it  an  angle  greater  than  NOP, 
is  an  ellipse.  The  less  the  angle  which  the  plane  of  the 
section  makes  with  the 
axis,  the  more  elongated 
is  the  ellipse. 

Any  section,  E  F,  of 
this  cone,  whose  plane 
makes  with  the  axis  an 
angle  equal  to  N  O  P,  is 
a  parabola.  It  will  be 
seen,  that,  by  changing  the 
obliquity  of  the  plane  CD 
to  the  axis  NO,  we  may 
pass  uninterruptedly  from 
the  circle  through  ellipses 
of  greater  and  greater 
elongation  to  the  parabola. 

Any  section,  GH,  of 
this  cone,  whose  plane 
makes  with  the  axis  ON 
an  angle  less  than  N  OP,  is  a  hyperbola. 

It  will  be  seen  from  Fig.  59,  in  which  comparative  views 
of  the  four  conic  sections  are  given,  that  the  circle  and 
the  ellipse  are  closed  curves,  or  curves  which  return  into 
themselves.  The  parabola  and  the  hyperbola  are,  on  the 
contrary,  open  curves,  or  curves  which  do  not  return  into 
themselves. 

49.  A  Revolving  Body  is  continually  Falling  towards  its 
Centre  of  Revolution.  —  In  Fig.  60  let  M  represent  the  moon, 


ASTRONOMY. 


and  E  the  earth  around  which  the  moon  is  revolving  in  the 
direction  M N.  It  will  be  seen  that  the  moon,  in  moving  from 
M  to  N,  falls  towards  the  earth  a  distance  equal  to  mN.  It 
is  kept  from  falling  into  the  earth  by  its  orbital  motion. 

The  fact  that  a 
body  might  be  pro- 
jected forward  fast 
enough  to  keep  it 
from  falling  into  the 
earth  is  evident  from 
Fig.  61.  A  B  repre- 
sents the  level  sur- 
face of  the  ocean, 
C  a  mountain  from 
the  summit  of  which 
a  cannon-ball  is  sup- 
posed to  be  fired  in 
the  direction  C  E. 
A  D  is  a  line  parallel 
with  CEj  DB  is  a 
line  equal  to  the  dis- 
tance between  the  two 
Fig"  59>  parallel  lines  A  D  and 

C  E.  This  distance  is  equal  to  that  over  which  gravity  would 
pull  a  ball  towards  the  centre  of  the  earth  in  a  minute.  No 
matter,  then,  with  what  velocity  the  ball  C  is  fired,  at  the  end 
of  a  minute  it  will  be  somewhere  on  the  line  A  D.  Suppose 
it  were  fired  fast  enough  to  reach  the  point  D  in  a  minute: 
it  would  be  on  the  line  A  D  at  the  end  of  the 
minute,  but  still  just  as  far  from  the  surface  of 
the  water  as  when  it  started.  It  will  be  seen, 
that,  although  it  has  all  the  while  been  falling 
towards  the  earth,  it  has  all  the  while  kept  at 
exactly  the  same  distance  from  the  surface. 
The  same  thing  would  of  course  be  true  dur- 
ing each  succeeding  minute,  till  the  ball  came 
round  to  C  again,  and  the  ball  would  continue  to  revolve  in  a 
circle  around  the  earth. 

50.  The  Form  of  a  Body's  Orbit  depends  upon  the  Rate  of 


Fig.  60. 


ASTRONOMY. 


its  Forward  Motion.  —  If  the  ball  C  were  fired  fast  enough  to 
reach  the  line  AD  beyond  the  point  D,  it  would  be  farther 
from  the  surface  at  the  end  of  the  second  than  when  it 
started.  Its  orbit  would  no  longer  be  circular,  but  ellipti- 


caL  If  the  speed  of  projection  were  gradually  augmented, 
the  orbit  would  become  a  more  and  more  elongated  ellipse. 
At  a  certain  rate  of  projection,  the  orbit  would  become  a 
parabola ;  at  a  still  greater  rate, 
a  hyperbola. 

51.  The    Moon    held   in    her 
Orbit  by  Gravity.  —  Newton  com- 
pared the  distance  inN  that  the 
moon  is   drawn   to   the  earth  in 
a  given  time,  with  the  distance 
a  body  near  the  surface  of   the 
earth    would    be    pulled   toward 
the  earth  in  the  same  time :  and 
he    found    that    these    distances 
are    to   each    other   inversely  as 
the  squares  of  the  distances  of 
the  two  bodies  from   the  centre 
of  the  earth.     He  therefore  con- 
cluded that  tJie  moon  is  drawn 

to   the   earth    by  gravity,   and    that   the    intensity   of  gravity 
decreases  as  the  square  of  the  distance  increases. 

52.  Any  Body  whose  Orbit  is  a   Conic  Section,  and  which 
moves  according  to  Kepler'' 's  Second  Law,  is  acted  upon  by  a 


ASTRONOMY. 


Force  varying  inversely  as  the  Square  of  the  Distance.  —  New- 
ton compared  the  distance  which  any  body, 
moving  in  an  ellipse,  according  to  Kepler's 
Second  Law,  is  drawn  towards  the  sun  in  the 
same  time  in  different  parts  of  its  orbit.  He 
found  these  distances  in  all  cases  to  vary 
ifiversely  as  the  square  of  the  distance  of 
the  planet  from  the  sun.  Thus,  in  Fig.  62, 
suppose  a  planet  would  move  from  K  to  B 
in  the  same  time  that  it  would  move  from  k 
to  b  in  another  part  of  its  orbit.  In  the  first 
instance  the  planet  would  be  drawn  towards 
the  sun  the  distance  A  B,  and  in  the  second 
instance  the  distance  ab.  Newton  found  that 
AB  :  ab-  SK2  :  Sk*.  He  also  found  that 
the  same  would  be  true  when  the  body  moved 
in  a  parabola  or  a  hyperbola:  hence  he  con- 
cluded that  every  body  that  moves  around  tJie 
sun  in  an  ellipse,  a  parabola,  or  a  liyperbola, 
is  moving  under  the  influence  of  gravity. 
53.  The  Force  that  draws  the  Different 

Planets   to   the   Sun    Varies   inversely  as  the  Squares  of  the 

Distances  of  the  Planets  from  the' Sun. —  Newton  compared 

the  distances  jK  and  eF, 

over  which  two  planets  are 

drawn  towards  the  sun  in 

the  same  time,  and  found 

these    distances    to    vary 

inversely   as    the    squares 

of    the    distances    of    the 

planets     from     the     sun  : 

hence  he  concluded   that 

all  the  planets   are  held 

in  their  orbits  by  gravity. 

He  also  showed  that  this 

would  be  true  of  any  two 

bodies  that  were  revolving 

around    the    sun's    centre,  *ig<  64< 

according  to  Kepler's  Third  Law. 


Fig.  63. 


ASTRONOMY.  53 

54.  The  Copernican  System. — The  theory  of  the  solar 
system  which  originated  with   Copernicus,  and  which  was 
developed  and  completed  by  Kepler  and  Newton,  is  com- 
monly known  as  the   Copernican  System.     This  system  is 
shown  in  Fig.  64. 

II.     THE    SUN   AND    PLANETS. 

I.     THE    EARTH. 

FORM  AND  SIZE. 

55.  Form    of  the    Earth.  —  In    ordinary   language     the 
term  horizon  denotes  the  line  that  bounds  the  portion    of 
the  earth's  surface  that  is  visible  at  any  point. 

(1)  It  is  well  known  that  the  horizon  of  a  plain  presents 
the    form   of   a   circle    surrounding    the    observer.     If    the 
latter  moves,  the  circle  moves  also ;  but  its  form  remains  the 
same,  and  is  modified  only  when  mountains  or  other  obsta- 
cles limit  the  view.     Out  at  sea,  the  circular  form  of  the 
horizon   is  still  more  decided,  and  changes  only  near  the 
coasts,  the  outline  of  which  breaks  the  regularity. 

Here,  then,  we  obtain  a  first  notion  of  the  rotundity  of 
the  earth,  since  a  sphere  is  the  only  body  which  is  presented 
always  to  us  under  the  form  of  a  circle,  from  whatever  point 
on  its  surface  it  is  viewed. 

(2)  Moreover,  it  cannot  be  maintained  that  the  horizon 
is  the  vanishing  point  of  distinct  vision,  and  that  it  is  this 
which  causes  the  appearance  of  a  circular  boundary,  because 
the  horizon  is  enlarged  when  we  mount  above  the   surface 
of  the  plain.     This  will  be  evident  from  Fig.  65,  in  which  a 
mountain  is  depicted  in  the  middle  of  a  plain,  whose  uni- 
form curvature  is  that  of  a  sphere.     From  the  foot  of  the 
mountain  the  spectator  will  have  but  a  very  limited  horizon. 
Let  him  ascend  half  way,  his  visual  radius  extends,  is  inclined 
below  the  first  horizon,  and  reveals  a  more  extended  circu- 


54 


ASTRONOMY. 


lar  area.     At  the  summit  of  the  mountain  the  horizon  still 
increases ;  and,  if  the  atmosphere  is  pure,  the  spectator  will 


Fig.  65. 

see  numerous  objects  where  from  the  lower  stations  the  sky 
alone  was  visible. 


Fig.  66. 


This  extension  of  the  horizon  would  be  inexplicable  if 
the  earth  had  the  form  of  an  extended  plane. 


ASTRONOMY.  55 

(3)  The  curvature  of  the  surface  of  the  sea  manifests 
itself  in  a  still  more  striking  manner.  If  we  are  on  the 
coast  at  the  summit  of  a  hill,  and  a  vessel  appears  on  the 
horizon  (Fig.  66),  we  see  only  the  tops  of  the  masts  and 
the  highest  sails ;  the  lower  sails  and  the  hull  are  invisible. 
As  the  vessel  approaches,  its  lower  part  comes  into  view 
above  the  horizon,  and  soon  it  appears  entire. 

In  the  same  manner  the  sailors  from  the  ship  see  the 
different  parts  of  objects  on  the  land  appear  successively, 
beginning  with  the  highest.  The  reason  of  this  will  be 
evident  from  Fig.  67,  where  the  course  of  a  vessel,  seen  in 
profile,  is  figured  on  the  convex  surface  of  the  sea. 

As  the  curvature  of  the  ocean  is  the  same  in  every  direc- 
tion, it  follows  that  the  surface  of  the  ocean  is  spherical. 


Fig.  67. 

The  same  is  true  of  the  surface  of  the  land,  allowance  being 
made  for  the  various  inequalities  of  the  surface.  From 
these  and  various  other  indications,  we  conclude  that  the 
earth  is  a  spJiere. 

56.  Size  of  the  Eartli. — The  size  of  the  earth  is  ascer- 
tained by  measuring  the  length  of  a  degree  of  a  meridian, 
and  multiplying  this  by  three  hundred  and  sixty.     This  gives 
the  circumference  of  the  earth  as  about  twenty-five  thousand 
miles,  and  its  diameter  as  about  eight  thousand  miles.     We 
know  that  the  two  stations  between  which  we  measure  are 
one  degree  apart  when  the  elevation  of  the  pole  at   one 
station  is  one  degree  greater  than  at  the  other. 

57.  The  Earth  Flattened  at  the  Poles.  —  Degrees  on  the 
meridian  have  been  measured  in  various  parts  of  the  earth, 
and  it  has  been  found  that  they  invariably  increase  in  length 


56  ASTRONOMY. 

as  we  proceed  from  the  equator  towards  the  pole  :  hence 
the  earth  must  curve  less  and  less  rapidly  as  we  approach  the 
poles ;  for  the  less  the  curvature  of  a  circle,  the  larger  the 
degrees  on  it. 

58.  The  Earth  in  Space.  —  In  Fig.  68  we  have  a  view 
of  the  earth  suspended  in  space.     The  side  of  the  earth 


turned  towards  the  sun  is  illumined,  and  the  other  side  is  in 
darkness.  As  the  planet  rotates  on  its  axis,  successive  por- 
tions of  it  will  be  turned  towards  the  sun.  As  viewed  from 
a  point  in  space  between  it  and  the  sun,  it  will  present 
light  and  dark  portions,  which  will  assume  different  forms 
according  to  the  portion  which  is  illumined.  These  differ- 
ent appearances  are  shown  in  Fig.  69. 


ASTRONOMY.  57 

DAY  AND  NIGHT. 
59.  Day  and  Night.  —  The  succession  of  day  and  night 


Fig.  69. 


is  due  to  the  rotation  of  the  earth  on  its  axis,  by  which  a 
place  on  the  surface  of  the  earth  is  carried  alternately  into 
the  sunshine  and  out  of  it.  As  the  sun  moves  around  the 


ASTRONOMY. 


heavens  on  the  ecliptic,  it  will  be  on  the  celestial  equator 
when  at  the  equinoxes,  and  23 |°  north  of  the  equator  when  at 
the  summer  solstice,  and  23^°  south  of  the  equator  when 

at  the  winter  solstice. 

60.  Day  and  Night 
when  the  Sun  is  at  the 
Equinoxes.  —  When  the 
sun  is  at  either  equinox, 
the  diurnal  circle  de- 
scribed by  the  sun  will 
coincide  with  the  celes- 
tial equator  :  and  there- 
fore half  of  this  diurnal 
circle  will  be  above  the 
horizon  at  every  point  on 
70.  the  surface  of  the  globe. 

At  these  times  day  and  night  will  be  equal  in  every  part  of 
the  eartJi. 


The  equality  of  days  and  nights  when  the  sun  is  on  the 
celestial  equator  is  also 
evident  from  the  following 
considerations :  one-half  of 
the  earth  is  in  sunshine  all 
of  the  time  :  when  the  sun 
is  on  the  celestial  equator, 
it  is  directly  over  the  equa- 
tor of  the  earth,  and  the 
illumination  extends  from 
pole  to  pole,  as  is  evident 
from  Figs.  70  and  71,  in 
the  former  of  which  the 
sun  is  represented  as  on 


Fig.  71. 


the    eastern    horizon    at    a 

place  along  the  central  line 

of    the    figure,   and    in    the    latter   as    on    the    meridian    along 

the  same  line.     In  each  diagram  it  is  seen  that  the  illumination 


ASTROXOMY. 


59 


extends  from  pole  to  pole :  hence,  as  the  earth  rotates  on  its 
axis,  every  place  on  the  surface  will  be  in  the  sunshine  and 
out  of  it  just  half  of  the  time. 

61.  Day  and  Night  when  tlie  Sun  is  at  the  Summer 
Solstice.  —  When  the  sun  is 
at  the  summer  solstice,  it 
will  be  23!°  north  of  the 
celestial  equator.  The  diur- 
nal circle  described  by  the 
sun  will  then  be  237^°  north 
of  the  celestial  equator ;  and 
more  than  half  of  this  diur- 
nal circle  will  be  above  the 
horizon  at  all  places  north 
of  the  equator,  and  less 
than  half  of  it  at  places  Fis-  v- 

south  of  the  equator  :  hence  the  days  will  be  longer  than  the 
nights  at  places  north  of  the  equator,  and  shorter  than  the 

nights  at  places  south  of 
the  equator.  At  places 
within  23^°  of  the  north 
pole,  the  entire  diurnal 
circle  described  by  the 
sun  will  be  above  the 
horizon,  so  that  the  sun 
will  not  set.  At  places 
within  23^°  of  the  south 
pole  of  the  earth,  the  en- 
tire diurnal  circle  will  be 
below  the  horizon,  so  that 
F»g-  73-  the  sun  will  not  rise. 

The  illumination  of  the  earth  at  this  time  is  shown  in 
Figs.  72  and  73.  In  Fig.  72  the  sun  is  represented  as  on  the 
western  horizon  along  the  middle  line  of  the  figure,  and  in 
Fig.  73  as  on  the  meridian.  It  is  seen  at  once  that  the  illu- 


6O  ASTRONOMY. 

mination  extends  23^°  beyond  the  north  pole,  and  falls  23^° 
short  of  the  south  pole.  As  the  earth  rotates  on  its  axis, 
places  near  the  north  pole  will  be  in  the  sunshine  all  the  time, 
while  places  near  the  south  pole  will  be  out  of  the  sunshine 
all  the  time.  All  places  north  of  the  equator  will  be  in  the 
sunshine  longer  than  they  are  out  of  it,  while  all  places  south 
of  "the  equator  will  be  out  of  the  sunshine  longer  than  they 
are  in  it. 

62.  Day  and  NigJit  when  tJie  Sun  is  at  the  Winter  Sol- 
stice.—  When  the  sun  is  at  the  winter  solstice,  it  is   23^° 
south  of  the  celestial  equator.     The  diurnal  circle  described 
by  the  sun  is  then  23!-°  south  of  the  celestial  equator.     More 
than  half  of  this  diurnal  circle  will  therefore  be  above  the 
horizon   at  all  places  south  of  the  equator,  and  less  than 
half  of  it  at  all  places  north  of  the  equator  :  hence  the  days 
will  be  longer  than   the   nights   south  of  the  equator,  and 
shorter  than  the  nights  at  places  north  of  the  equator.     At 
places  within  23!°  of  the  south  pole,  the  diurnal  circle  de- 
scribed by  the  sun  will  be  entirely  above  the  horizon,  and 
the  sun  will  therefore  not  set.     At  places  within  23^°  of  the 
north  pole,  the  diurnal  circle  described  by  the  sun  will  be 
wholly  below  the  horizon,  and  therefore  the  sun  will  not  rise. 

The  illumination  of  the  earth  at  this  time  is  shown  in 
Figs.  74  and  75,  and  is  seen  to  be  the  reverse  of  that  shown 
in  Figs.  72  and  73. 

63.  Variation    in    the  Length  of  Day  and  Night.  —  As 
long  as  the  sun  is  north  of  the  equinoctial,  the  nights  will 
be  longer  than  the  days  south  of  the  equator,  and  shorter 
than  the  days  north  of  the  equator.     It  is  just  the  reverse 
when  the  sun  is  south  of  the  equator. 

The  farther  the  sun  is  from  the  equator,  the  greater  is  the 
inequality  of  the  days  and  nights. 

The  farther  the  place  is  from  the  equator,  the  greater  the 
inequality  of  its  days  and  nights. 

When  the  distance  of  a  place  from  the  north  pole  is  less 


ASTRONOMY.  6 1 

than  the  distance  of  the  sun  north  of  the  equinoctial,  it 
will  have  continuous  day  without  night.,  since  the  whole  of 
the  sun's  diurnal  circle  will  be  above  the  horizon.  A  place 
within  the  same  distance  of 
the  south  pole  will  have 
continuous  night. 

When  the  distance  of  a 
place  from  the  north  pole  is 
less  than  the  distance  of  the 
sun  south  of  the  equinoc- 
tial, it  will  have  continuous 
night,  since  the  whole  of 
the  sun's  diurnal  circle  will 
then  be  below  the  horizon. 
A  place  within  the  same  Flg-  74- 

distance  of  the  south  pole  will  then  have  continuous  day. 

At  the  equator  the  days  and  nights  are  always  equal; 
since,  no  matter  where  the  sun  is  in  the  heavens,  half  of 
all  the  diurnal  circles  described  by  it  will  be  above  the 

horizon,  and  half  of  them 
below  it. 

64.  The  Zones.  —  It 
will  be  seen,  from  what 
has  been  stated  above, 
that  the  sun  will  at  some 
time  during  the  year  be 
directly  overhead  at  every 
place  within  23^°  of  the 
equator  on  either  side. 
This  belt  of  the  earth  is 
called  the  torrid  zone. 
Fis-  75-  The  torrid  zone  is  bound- 

ed   by  circles   called   the   tropics ;    that  of  Cancer  on   the 
north,  and  that  of  Capricorn  on  the  south. 

It  will  also  be  seen,  that,  at  every  place  within   23^°  of 


62  ASTRONOMY. 

either  pole,  there  will  be,  some  time  during  the  year,  a 
day  during  which  the  sun  will  not  rise,  or  on  which  it  will 
not  set.  These  two  belts  of  the  earth's  surface  are  called 
the  frigid  zones.  These  zones  are  bounded  by  the  arctic 
circles.  The  nearer  a  place  is  to  the  poles,  the  greater  the 
number  of  days  on  which  the  sun  does  not  rise  or  set. 

Between  the  frigid  zones  and  the  torrid  zones,  there  are 
two  belts  on  the  earth  which  are  called  the  temperate  zones. 
The  sun  is  never  overhead  at  any  place  in  these  two  zones, 
but  it  rises  and  sets  every  day  at  every  place  within  their 
limits. 

65.  The   Width  of  the  Zones.  —  The  distance  the  frigid 
zones  extend  from  the  poles,  and  the  torrid  zones  from  the 
equator,  is  exactly  equal  to  the  obliquity  of  the  ecliptic,  or  the 
deviation  of  the  axis  of  the  earth  from  the  perpendicular  to 
the  plane  of  its  orbit.    Were  this  deviation  forty-five  degrees, 
the  obliquity  of  the  ecliptic  would  be  forty-five  degrees,  the 
torrid  zone  would  extend  forty-five  degrees  from  the  equator, 
and  the  frigid  zones  forty-five  degrees  from  the  poles.     In 
this  case  there  would  be  no  temperate  zones.     Were  this 
deviation  fifty  degrees,  the  torrid  and   frigid   zones  would 
overlap  ten  degrees,  and  there  would  be  two  belts  of  ten 
degrees  on   the   earth,  which  would  experience  alternately 
during  the  year  a  torrid  and  a  frigid  climate. 

Were  the  axis  of  the  earth  perpendicular  to  the  plane 
of  the  earth's  orbit,  there  would  be  no  zones  on  the  earth, 
and  no  variation  in  the  length  of  day  and  night. 

66.  Twilight.  —  Were    it    not    for   the    atmosphere,    the 
darkness    of  midnight  would   begin    the    moment  the   sun 
sank  below  the    horizon,  and  would  continue  till  he  rose 
again  above  the  horizon  in  the  east,  when  the  darkness  of 
the    night  would    be   suddenly  succeeded   by  the  full  light 
of  day.     The   gradual   transition  from  the  light  of  day  to 
the  darkness  of  the  night,   and  from  the   darkness  of  the 
night  to  the  light  of  day,  is  called  twilight,  and  is  due  to 


ASTRONOMY.  63 

the  diffusion  of  light  from  the  upper  layers  of  the  atmos- 
phere after  the  sun  has  ceased  to  shine  on  the  lower  layers 
at  night,  or  before  it  has  begun  to  shine  on  them  in  the 
morning. 

\^ABCD  (Fig.  76)  represent  a  portion  of  the  earth, 
A  a  point  on  its  surface  where  the  sun  -S  is  setting ;  and  let 
SAH  be  a  ray  of  light  just  grazing  the  earth  at  A,  and 
leaving  the  atmosphere  at  the  point  H.  The  point  A  is 
illuminated  by  the  whole  reflective  atmosphere  HGFE. 
The  point  B,  to  which  the  sun  has  set,  receives  no  direct 


Fig.  76. 

solar  light,  nor  any  reflected  from  that  part  of  the  atmos- 
phere which  is  below  ALH ;  but  it  receives  a  twilight  from 
the  portion  HLF,  which  lies  above  the  visible  horizon  B F. 
The  point  C  receives  a  twilight  only  from  the  small  portion 
of  the  atmosphere  HMG;  while  at  D  the  twilight  has 
ceased  altogether. 

67.  Duration  of  Twilight  —  The  astronomical  limit  of 
twilight  is  generally  understood  to  be  the  instant  when  stars 
of  the  sixth  magnitude  begin  to  be  visible  in  the  zenith  at 
evening,  or  disappear  in  the  morning. 

Twilight  is  usually  reckoned  to  last  until  the  sun's  depres- 
sion below  the  horizon  amounts  to  eighteen  degrees  :  this,  how- 
ever, varies ;  in  the  tropics  a  depression  of  sixteen  or  seventeen 
degrees  being  sufficient  to  put  an  end  to  the  phenomenon, 
while  in  England  a  depression  of  seventeen  to  twenty-one 
degrees  is  required.  The  duration  of  twilight  differs  in  differ- 


64  ASTRONOMY. 

ent  latitudes;  it  varies  also  in  the  same  latitude  at  different 
seasons  of  the  year,  and  depends,  in  some  measure,  on  the 
meteorological  condition  of  the  atmosphere.  When  the  sky 
is  of  a  pale  color,  indicating  the  presence  of  an  unusual 
amount  of  condensed  vapor,  twilight  is  of  longer  duration. 
This  happens  habitually  in  the  polar  regions.  On  the  contrary, 
within  the  tropics,  where  the  air  is  pure  and  dry,  twilight  some- 
times lasts  only  fifteen  minutes.  Strictly  speaking,  in  the  lati- 
tude of  Greenwich  there  is  no  true  night  from  May  22  to 
July  21,  but  constant  twilight  from  sunset  to  sunrise.  Twilight 
reaches  its  minimum  three  weeks  before  the  vernal  equinox, 
and  three  weeks  after  the  autumnal  equinox,  when  its  duration 
is  an  hour  and  fifty  minutes.  At  midwinter  it  is  longer  by 
about  seventeen  minutes  ;  but  the  augmentation  is  frequently 
not  perceptible,  owing  to  the  greater  prevalence  of  clouds  and 
haze  at  that  season  of  the  year,  which  intercept  the  light,  and 
hinder  it  from  reaching  the  earth.  The  duration  is  least  at 
the  equator  (an  hour  and  twelve  minutes),  and  increases  as 
we  approach  the  poles;  for  at  the  former  there  are  two  twi- 
lights every  twenty-four  hours,  but  at  the  latter  only  two  in  a 
year,  each  lasting  about  fifty  days.  At  the  north  pole  the  sun 
is  below  the  horizon  for  six  months,  but  from  Jan.  29  to  the 
vernal  equinox,  and  from  the  autumnal  equinox  to  Nov.  12, 
the  sun  is  less  than  eighteen  degrees  below  the  horizon:  so 
that  there  is  twilight  during  the  whole  of  these  intervals,  and 
thus  the  length  of  the  actual  night  is  reduced  to  two  months 
and  a  half.  The  length  of  the  day  in  these  regions  is  about 
six  months,  during  the  whole  of  which  time  the  sun  is  con- 
stantly above  the  horizon.  The  general  rule  is,  that  to  the 
inhabitants  of  an  obliqiie  sphere  the  twilight  is  longer  in  pro- 
portion as  the  place  is  nearer  the  elevated  pole,  and  the  sun  is 
farther  front  the  equator  on  the  side  of  the  elevated  pole. 

THE  SEASONS. 

68.  The  Seasons.  —  While  the  sun  is  north  of  the  celes- 
tial equator,  places  north  of  the  equator  are  receiving  heat 
from  the  sun  by  day  longer  than  they  are  losing  it  by  radia- 
tion at  night,  while  places  south  of  the  equator  are  losing 


ASTRONOMY. 


heat  by  radiation  at  night  longer  than  they  are  receiving  it 
from  the  sun  by  day.  When,  therefore,  the  sun  passes  north 
of  the  equator,  the  temperature  begins  to  rise  at  places 
north  of  the  equator,  and  to  fall  at  places  south  of  it.  The 
rise  of  temperature  is  most  rapid  north  of  the  equator  when 
the  sun  is  at  the  summer  solstice ;  but,  for  some  time  after 
this,  the  earth  continues  to  receive  more  heat  by  day  than  it 
loses  by  night,  and  therefore  the  temperature  continues  to 
rise.  For  this  reason,  the  heat  is  more  excessive  after  the 
sun  passes  the  summer  solstice  than  before  it  reaches  it. 

69.  The  Duration  of  tlic  Seasons.  —  Summer  is  counted 
as  beginning  in  June,  when  the  sun  is  at  the  summer  sol- 
stice, and  as  continuing  until  the  sun  reaches  the  autumnal 
equinox,  in  September.  Autumn  then  begins,  and  continues 
until  the  sun  is  at  the  winter  solstice,  in  December.  Winter 
follows,  continuing  until  the  sun  comes  to  the  vernal  equinox, 
in  March,  when  spring 
begins,  and  continues 
to  the  summer  sol- 
stice. In  popular 
reckoning  the  sea- 
sons begin  with  the 
first  day  of  June, 
September,  Decem- 
ber, and  March. 

The  reason  why 
winter  is  counted  as 
occurring  after  the 
winter  solstice  is  simi- 
lar to  the  reason  why 
the  summer  is  placed 
after  the  summer  solstice.  The  earth  north  of  the  equator 
is  losing  heat  most  rapidly  at  the  time  of  the  winter  solstice  ; 
but  for  some  time  after  this  it  loses  more  heat  by  night  than 
it  receives  by  day :  hence  for  some  time  the  temperature 


66 


ASTRONOMY. 


continues   to    fall,  and   the  cold  is  more  intense    after  the 
winter  solstice  than  before  it. 


Of  course,  when  it  is  summer  in  the  northern  hemisphere, 
it   is   winter  in   the  southern  hemisphere,  and  the  reverse. 


ASTRONOMY. 


Fig.  77  shows  the  portion  of  the  earth's  orbit  included  in 

each  season.     It  will  be  seen  that  the  earth  is  at  perihelion 

in  the  winter  season  for 

places     north     of    the 

equator,  and  at  aphelion 

in  the  summer  season. 

This   tends  to   mitigate 

somewhat  the   extreme 

temperatures      of     our 

winters  and  summers. 

70.  Tli  c  I/I  if  m  in  a  tio  n 
of   the   Earth    at    the 
different       Seasons.  — 
Fig.  78  shows  the  earth 
as    it  would    appear   to 

an  observer  at  the  sun  F'g-  79. 

during  each  of  the  four  seasons  ;  that  is  to  say,  the  por- 
tion of  the  earth  that  is  receiving  the  sun's  rays.  Figs.  79, 
80,  81,  and  82  are  enlarged  views  of  the  earth,  as  seen 

from  the  sun  at  the  time 
of  the  summer  solstice,  of 
the  autumnal  equinox,  of 
the  winter  solstice,  and 
of  the  vernal  equinox. 

Fig.  83  is,  so  to  speak, 
a  side  view  of  the  earth, 
showing  the  limit  of  sun- 
shine on  the  earth  when 
the  sun  is  at  the  summer 
solstice ;  and  Fig.  84. 
showing  the  limit  of  sun- 
shine when  the  sun  is  at 
the  autumnal  equinox. 

71.  Cause  of  the   Change  of  Seasons. — Variety  in  the 
length  of  day  and  night,  and  diversity  in  the  seasons,  depend 


Fig.  80. 


68 


ASTRONOMY. 


upon  the  obliquity  of  the  ecliptic. 


Fig.  8x. 


Were  there  no  obliquity 
of  the  ecliptic,  there 
would  be  no  inequality 
in  the  length  of  day 
and  night,  and  but 
slight  diversity  of  sea- 
sons. The  greater  the 
obliquity  of  the  eclip- 
tic, the  greater  would 
be  the  variation  in  the 
length  of  the  days  and 
nights,  and  the  more 
extreme  .  the  changes 
of  the  seasons. 


TIDES. 

72.  Tides.  —  The  alternate  rise  and  fall  of  the  surface  of 
the  sea  twice  in  the  course  of  a  lunar  day,  or  of  twenty-four 
hours  and  fifty-one  minutes,  is  known  as  the  tides.  When 
the  water  is  rising,  it  is  said  to  be  flood  tide  ;  and  when 
it  is  falling,  ebb  tide. 
When  the  water  is  at  its 
greatest  height,  it  is  said 
to  be  high  water ;  and 
when  at  its  least  height, 
low  water. 


73.  Cause  of  the  Tides. 
—  It  has  been  known  to 
seafaring  nations  from  a 
remote  antiquity  that  there 
is  a  singular  connection 
between  the  ebb  and  flow 
of  the  tides  and  the  diur- 
nal motion  of  the  moon. 

This  tidal  movement  in  seeming  obedience  to  the  moon  was 
a  mystery  until  the  study  of  the  law  of  gravitation  showed  it 


ASTRONOMY. 


69 


to  be  due  to  the  attraction  of  the  moon  on  the  waters  of  the 
ocean.     The  reason  why  there  are  two  tides  a  day  will  appear 


from  Fig.  85.  Let  M  be  the  moon,  E  the  earth,  and  EM  the 
line  joining  their  centres.  Now,  strictly  speaking,  the  moon 
does  not  revolve  around  the  earth  anv  more  than  the  earth 


Fig.  84. 

around  the  moon ;  but  the  centre  of  each  body  moves  around 
the  common  centre  of  gravity  of  the  two  bodies.     The  earth 


7O  ASTRONOMY. 

being  eighty  times  as  heavy  as  the  moon,  this  centre  is  situated 
within  the  former,  about  three-quarters  of  the  way  from  its 
centre  to  its  surface,  at  the  point  G.  The  body  of  the  earth 
itself  being  solid,  every  part  of  it,  in  consequence  of  the 
moon's  attraction,  may  be  considered  as  describing  a  circle 
once  in  a  month,  with  a  radius  equal  to  EG.  The  centrifugal 
force  caused  by  this  rotation  is  just  balanced  by  the  mean 
attraction  of  the  moon  upon  the  earth.  If  this  attraction  were 
the  same  on  every  part  of  the  earth,  there  would  be  even- 
where  an  exact  balance  between  it  and  the  centrifugal  force. 
But  as  we  pass  from  E  to  D  the  attraction  of  the  moon  dimin- 
ishes, owing  to  the  increased  distance :  hence  at  D  the  centri- 
fugal force  predominates,  and  the  water  therefore  tends  to  move 
away  from  the  centre  E'.  As  we  pass  from  E  towards  C,  the 
attraction  of  the  moon  increases,  and  therefore  exceeds  the  ten- 


Fig.  85. 

trifugal  force  :  consequently  at  C  there  is  a  tendency  to  draw 
the  water  towards  the  moon,  but  still  away  from  the  centre  E. 
At  A  and  B  the  attraction  of  the  moon  increases  the  gravity 
of  the  water,  owing  to  the  convergence  of  the  lines  B M  and 
AM,  along  which  it  acts:  hence  the  action  of  the  moon  tends 
to  make  the  waters  rise  at  D  and  C,  and  to  fall  at  A  and  B, 
causing  two  tides  to  each  apparent  diurnal  revolution  of  the 
moon. 

74.  The  Lagging  of  the  Tides. —  If  the  waters  everywhere 
yielded  immediately  to  the  attractive  force  of  the  moon,  it  would 
always  be  high  water  when  the  moon  was  on  the  meridian,  low 
water  when  she  was  rising  or  setting,  and  high  water  again 
when  she  was  on  the  meridian  below  the  horizon.  But,  owing 
to  the  inertia  of  the  water,  some  time  is  necessary  for  so  slight 
a  force  to  set  it  in  motion ;  and,  once  in  motion,  it  continues 
so  after  the  force  has  ceased,  and  until  it  has  acted  some  time 
in  the  opposite  direction.  Therefore,  if  the  motion  of  the 


ASTRONOMY.  /I 

water  were  unimpeded,  it  would  not  be  high  water  until  some 
hours  after  the  moon  had  passed  the  meridian.  The  free 
motion  of  the  water  is  also  impeded  by  the  islands  and  conti- 
nents. These  deflect  the  tidal  wave  from  its  course  in  such  a 


way  that  it  may,  in  some  cases,  be  many  hours,  or  even  a 
whole  day,  behind  its  time.  Sometimes  two  waves  meet  each 
other,  and  raise  a  very  high  tide.  In  some  places  the  tides 


Fig.  87. 


run  up  a  long  bay,  where  the  motion  of  a  large  mass  of  water 
will  cause  an  enormous  tide  to  be  raised.  In  the  Bay  of 
Fundy  both  of  these  causes  are  combined.  A  tidal  wave  com- 
ing up  the  Atlantic  coast  meets  the  ocean  wave  from  the 
east,  and,  entering  the  bay  with  their  combined  force,  they 


/2  ASTRONOMY. 

raise  the  water  at  the    head   of  it   to    the    height  of   sixty  or 
seventy  feet. 

75.  Spring-Tides  and  Neap-Tides.  —  The  sun  produces 
a  tide  as  well  as  the  moon ;  but  the  tide-producing  force 
of  the  sun  is  only  about  four-tenths  of  that  of  the  moon. 
At  new  and  full  moon  the  two  bodies  unite  their  forces, 
the  ebb  and  flow  become  greater  than  the  average,  and 
we  have  the  spring-tides.  When  the  moon  is  in  her  first 


Fig.  88. 

or  third  quarter,  the  two  forces  act  against  each  other ; 
the  tide-producing  force  is  the  difference  of  the  two ;  the 
ebb  and  flow  are  less  than  the  average  ;  and  we  have  the 
neap-tides. 

Fig.  86  shows  the  tide  that  would  be  produced  by  the 
moon  alone;  Fig.  87,  the  tide  produced  by  the  combined 
action  of  the  sun  and  moon  ;  and  Fig.  88,  by  the  sun  and 
moon  acting  at  right  angles  to  each  other. 

The  tide  is  affected  by  the  distance  of  the  moon  from 


ASTRONOMY. 


73 


the  earth,  being  highest  near  the  time  when  the  moon  is  in 
perigee,  and  lowest  near  the  time  when  she  is  in  apogee. 
When  the  moon  is  in  perigee,  at  or  near  the  time  of  a  new 
or  full  moon,  unusually  high  tides  occur. 

76.  Diurnal  Inequality  of  Tides.  —  The  height  of  the  tide 
at  a  given  place  is  influenced  by  the  declination  of  the  moon. 
When  the  moon  has  no  declination,  the  highest  tides  should 
occur  along  the  equator,  and  the  heights  should  diminish  from 
thence  toward  the  north  and  south ;  but  the  two  daily  tides  at 
any  place  should  have  the  same  height.     When  the  moon  has 
north  declination,  as   shown  in   Fig.  89,  the  highest  tides  on 
the  side  of  the  earth  next  the  moon  will  be  at  places  having 
a  corresponding 

north  latitude, 
as  at  B,  and  on 
the  opposite 
side  at  those 
which  have  an 
equal  south  lati- 
tude. Of  the 
two  daily  tides 
at  any  place,  that 
which  occurs 
when  the  moon 
is  nearest  the 

zenith  should  be  the  greatest :  hence,  when  the  moon's  declina- 
tion is  north,  the  height  of  the  tide  at  a  place  in  north  latitude 
should  be  greater  when  the  moon  is  above  the  horizon  than 
when  she  is  below  it.  At  the  same  time,  places  south  of  the 
equator  have  the  highest  tides  when  the  moon  is  below  the 
horizon,  and  the  least  when  she  is  above  it.  This  is  called 
the  diurnal  inequality,  because  its  cycle  is  one  day;  but  it 
varies  greatly  in  amount  at  different  places. 

77.  HeigJit  of  Tides.  —  At  small    islands   in    mid-ocean 
the  tides  never  rise  to  a  great  height,  sometimes  even  less 
than  one  foot ;  and  the  average  height  of  the  tides  for  the 
islands  of  the  Atlantic  and  Pacific  Oceans  is  only  three  feet 


74  ASTRONOMY. 

and  a  half.  Upon  approaching  an  extensive  coast  where 
the  water  is  shallow,  the  height  of  the  tide  is  increased ;  so 
that,  while  in  mid-ocean  the  average  height  does  not  exceed 
three  feet  and  a  half,  the  average  in  the  neighborhood  of 
continents  is  not  less  than  four  or  five  feet. 

THE  DAY  AND  TIME. 

78.  The  Day.  —  By  the  term   day  we  sometimes  denote 
the  period  of  sunshine  as  contrasted  with  that  of  the  absence 
of  sunshine,  which  we  call  night,  and  sometimes  the  period 
of  the   earth's  rotation   on   its  axis.     It  is  with  the  latter 
signification  that  the  term  is  used  in  this  section.     As  the 
earth  rotates  on  its  axis,  it  carries  the  meridian  of  a  place 
with  it ;  so  that,  during  each  complete  rotation  of  the  earth, 
the  portion   of  the   meridian  which  passes  overhead  from 
pole  to  pole  sweeps  past  every  star  in  the  heavens  from  west 
to  east.     The  interval  between  two  successive  passages  of 
this  portion   of  the  meridian  across  the  same  star  is  the 
exact  period  of  the  complete  rotation  of  the  earth.     This 
period  is  called  a  sidereal  day.     The  sidereal  day  may  also 
be  defined  as  the  interval  between  two  successive  passages 
of  the  same  star  across  the  meridian  ;  the  passage  of  the 
meridian  across  the  star,  and  the  passage  or  transit  of  the 
star  across  the  meridian,  being  the  same  thing  looked   at 
from  a  different  point  of  view.     The  interval  between  two 
successive  passages  of  the  meridian  across  the  sun,  or  of  tJie 
sun  across  the  meridian,  is  called  a  solar  day. 

79.  Length  of  the  Solar  Day.  —  The  solar  day  is  a  little 
longer  than  the  sidereal  day.     This  is  owing  to  the   sun's 
eastward  motion  among  the  stars.     We  have  already  seen 
that  the  sun's  apparent  position  among  the  stars  is  continu- 
ally shifting  towards  the  east  at  a  rate  which  causes  it  to 
make  a  complete  circuit  of  the  heavens  in  a  year,  or  three 
hundred  and  sixty-five  days.     This  is  at  the  rate  of  about 
one  degree  a  day  :  hence,  were  the  sun  and  a  star  on  the 


ASTRONOMY. 


75 


meridian  together  to-day,  when  the  meridian  again  came 
around  to  the  star,  the  sun  would 
appear  about  one  degree  to  the 
eastward  :  hence  the  meridian  must 
be  carried  about  one  degree  far- 
ther in  order  to  come  up  to  the 
sun.  The  solar  day  must  there- 
fore be  about  four  minutes  longer 
than  the  sidereal  day. 

The  fact  that  the  earth  must 
make  more  than  a  complete  rota- 
tion is  also  evident  from  Figs.  90 
and  91.  In  Fig.  90,  ba  represents 
the  plane  of  the  meridian,  and  the 
small  arrows  indicate  the  direction 
the  earth  is  rotating  on  its  axis, 
and  revolving  in  its  orbit.  When 


Fig.  90. 


the  earth  is  at  i,  the  sun  is  on  the  meridian  at  a.     When 


Fig.  91. 

the  earth  has  moved  to  2,  it  has  made  a  complete  rotation, 
as  is  shown  by  the  fact  that  the  plane  of  the  meridian  is 


76 


ASTRONOMY. 


parallel  witli  its  position  at  i  ;  but  it  is  evident  that  the 
meridian  has  not  yet  come  up  with  the  sun.  In  Fig.  91, 
OA  represents  the  plane  of  the  meridian,  and  OS  the 
direction  of  the  sun.  The  small  arrows  indicate  the  direc- 
tion of  the  rotation  and 
revolution  of  the  earth.  In 
passing  from  the  first  posi- 
tion to  the  second  the 
earth  makes  a  complete 
rotation,  but  the  meridian 
is  not  brought  up  to  the 
sun. 

80.  Inequality  in  the 
Length  of  Solar  Days.  — 
The  sidereal  days  are  all 
of  the  same  length  ;  but  the 
solar  days  differ  somewhat  in  length.  This  difference  is  due 
to  the  fact  that  the  sun's  apparent  position  moves  eastward, 
or  away  from  the  meridian,  at  a  variable  rate. 

There  are  three  reasons  why  this  rate  is  variable  :  — 

(1)  The  sun's  eastward  motion  is 
due  to  the  revolution  of  the  earth 
in  its  orbit.    Now,  the  earth's  orbital 
motion  is  not  uniform,  being  fast- 
est when  the  earth  is  at  perihelion, 
and   slowest  when   the    earth  is  at 
aphelion  :  hence,  other  things  being 
equal,   solar   days    will  be    longest 
when    the    earth    is    at    perihelion, 
and  shortest  when  the  earth  is  at 
aphelion. 

(2)  The   sun's  eastward   motion 

is  along  the  ecliptic.  Now,  from  Figs.  92  and  93,  it  will  be 
seen,  that,  when  the  sun  is  at  one  of  the  equinoxes,  it  will 
be  moving  away  from  the  meridian  obliquely;  and,  from  Figs. 
94  and  95,  that,  when  the  sun  is  at  one  of  the  solstices,  it  will 


Fig-  93- 


ASTRONOMY. 


77 


be  moving  away  from  the  meridian  perpendicularly  :  hence, 

other  things  being  equal,  the  sun  would  move  away  from  the 

meridian  fastest,  and  the  days  be  longest,  when  the  sun  is  at 

the  solstices;   while  it  would  move  away  from   the  meridian 

slowest,  and  the  days  be  shortest,  when  the  sun  is  at  the  equi- 

noxes. -  That  a  body  moving 

along  the   ecliptic    must    be 

moving  at  a  variable  angle  to 

the    meridian    becomes    very 

evident  on  turning  a  celestial 

globe    so   as    to   bring  each 

portion  of  the  ecliptic  under 

the  meridian  in  turn. 

(3)  The  sun,  moving  along 

the  ecliptic,  always  moves  in 

a  great  circle,  while  the  point 

of  the   meridian  which  is  to 

overtake  the  sun  moves  in  a 

diurnal  circle,  which  is  some- 

tintfs  a  great  circle  and  sometimes  a  small  circle.     When  the 

sun  is  at  the  equinoxes,  the  point  of  the  meridian  which  is  to 

overtake  it  moves  in  a  great  circle.     As  the  sun  passes  from 

the  equinoxes  to  the  solstices, 
the  point  of  the  meridian  which 
is  to  overtake  it  moves  on  a 
smaller  and  smaller  circle  :  hence, 
as  we  pass  away  from  the  celes- 
-  -  tial  equator,  the  points  of  the 
meridian  move  slower  and  slower. 
Therefore,  other  things  being 
equal,  the  meridian  will  gain 
upon  the  sun  most  rapidly,  and 
the  days  be  shortest,  when  the 
sun  is  at  the  equinoxes  ;  while  it 


Fig-  95- 


will  gain  on  the  sun  least  rapidly,  and  the  days  will  be  longest, 
when  the  sun  is  at  the  solstices. 

The  ordinary  or  civil  day  is  the  mean  of  all  the  solar 
days  in  a  year. 


78  ASTRONOMY. 

81.  Sun  Time  and  Clock  Time.  —  It  is  noon  by  the  sun 
when  the  sun  is  on  the  meridian,  and  by  the  clock  at  the 
middle  of  the  civil  day.     Now,  as  the  civil  days  are  all  of 
the  same  length,  while  solar  days  are  of  variable  length,  it 
seldom  happens  that  the  middles  of  these  two  days  coincide, 
or  that   sun   time   and  clock  time  agree.     The   difference 
between  sun  time  and  clock  time,  or  what  is  often  called 
apparent  solar  time  and  mean  solar  time,  is  called  the  equa- 
tion of  time.     The  sun  is  said  to  be  slow  when  it  crosses  the 
meridian  after  noon  by  the  clock,  and  fast  when  it  crosses 
the  meridian  before  noon  by  the  clock.     Sun  time  and  clock 
time  coincide  four  times  a  year ;  during  two  intermediate 
seasons  the  clock  time  is  ahead,  and  during  two  it  is  behind. 

The  following  are  the  dates  of  coincidence  and  of  maxi- 
mum deviation,  which  vary  but  slightly  from  year  to  year:  — 

February  10     ...  True  sun  fifteen  minutes  slow. 

April  15 True  sun  correct. 

May  14    .....  True  sun  four  minutes  fast. 

June  14 True  sun  correct. 

July  25 True  sun  six  minutes  slow. 

August  31     ....  True  sun  correct. 

November  2     ...  True  sun  sixteen  minutes  fast. 

December  24    ...  True  sun  correct. 

One  of  the  effects  of  the  equation  of  time  which  is  fre- 
quently misunderstood,  is,  that  the  interval  from  sunrise  until 
noon,  as  given  in  the  almanacs,  is  not  the  same  as  that  between 
noon  and  sunset.  The  forenoon  could  not  be  longer  or  shorter 
than  the  afternoon,  if  by  "  noon  "  we  meant  the  passage  of  the 
sun  across  the  meridian  ;  but  the  noon  of  our  clocks  being 
sometimes  fifteen  minutes  before  or  after  noon  by  the  sun,  the 
former  may  be  half  an  hour  nearer  to  sunrise  than  to  sunset, 
or  "vice  -versa. 

THE  YEAR. 

82.  The   Year.  —  The  year  is  the  time  it  takes  the  earth 
to  revolve  around  the  sun,  or,  what  amounts  to  the  same 
thing,  the  time  it  takes  the  sun  to  pass  around  the  ecliptic. 


ASTRONOMY.  79 

1 i )  The  time  it  takes  the  sun  to  pass  from  a  star  around 
to  the  same  star  again  is  called  a  sidereal  year.     This  is, 
of  course,  the  exact  time  it  takes  the  earth  to  make  a  com- 
plete revolution  around  the  sun. 

(2)  The  time  it  takes  the  sun  to  pass  around  from  the 
vernal    equinox,   or   the  first 

point  of  Aries,  to  the  vernal 

equinox    again,  is  called  the 

tropical  year.     This  is  a  little 

shorter  than  the  sidereal  year, 

owing    to    the    precession    of 

the  equinoxes.     This  will  be 

evident   from   Fig.   96.     The 

circle  represents  the  ecliptic, 

.S  the  sun,  and  E  the  vernal 

equinox.      The     sun     moves 

around  the  eh'ptic  eastward,  as  indicated  by  the  long  arrow, 

while  the  equinox  moves  slowly  westward,  as  indicated  by 

the  short  arrow,     The  sun  will  therefore  meet  the  equinox 

before  it  has  quite  completed  the  circuit  of  the  heavens. 

The  exact  lengths  of  these  respective  years  are  :  — 

DAYS.  HOURS.     MIX.     SEC. 

Sidereal  year      .     .  365.25636=365         699 
Tropical  year     .     .  365. 24220= 365         5       48       46 

Since  the  recurrence  of  the  seasons  depends  on  the  tropi- 
cal year,  the  latter  is  the  one  to  be  used  in  forming  the 
calendar  and  for  the  purposes  of  civil  life  generally.  Its 
true  length  is  eleven  minutes  and  fourteen  seconds  less  than 
three  hundred  and  sixty-five  days  and  a  fourth. 

It  will  be  seen  that  the  tropical  year  is  about  twenty  min- 
utes shorter  than  the  sidereal  year. 

(3)  The  time  it  takes  the  earth  to  pass  from  its  perihelion 
point  around  to  the  perihelion  point  again  is  called  the  anoma- 
listic year.     This  year  is  about  four  minutes  longer  than  the 
sidereal  year.     This  is  owing  to  the  fact  that  the  major  axis  of 


8O  ASTRONOMY. 

the  earth's  orbit  is  slowly  moving  around  to  the  east  at  the 
rate  of  about  ten  seconds  a  year.  This  causes  the  perihelion 
point  P  (Fig.  97)  to  move  eastward  at  that  rate,  as  indicated 
by  the  short  arrow.  The  earth  E  is  also  moving  eastward,  as 
indicated  by  the  long  arrow.  Hence  the  earth,  on  starting  at 
the  perihelion,  has  to  make  a  little  more  than  a  complete  circuit 
to  reach  the  perihelion  point  again. 

83.  The  Calendar.  —  The  solar  year,  or  the  interval  between 
two  successive  passages  of  the  same  equinox  by  the  sun, 

is  365  days,  5  hours,  48  min- 
utes, 46  seconds.  If,  then, 
we  reckon  only  365  days  to 
a  common  or  civil  year,  the 
sun  will  come  to  the  equinox 

?  ^  hours,  48  minutes,  46  sec- 

\\\  p  /    onds,  or  nearly  a  quarter  of 

Y^  /     a   day,   later   each   year;    so 

that,  if  the  sun  entered  Aries 
on  the   2oth    of    March    one 
year,  he    would    enter   it   on 
Flg<  97>  the  2  ist  four  years  after,  on 

the  22d  eight  years  after,  and  so  on.  Thus  in  a  comparatively 
short  time  the  spring  months  would  come  in  the  winter,  and 
the  summer  months  in  the  spring. 

Among  different  ancient  nations  different  methods  of  com- 
puting the  year  were  in  use.  Some  reckoned  it  by  the  revo- 
lution of  the  moon,  some  by  that  of  the  sim ;  but  none,  so 
far  as  we  know,  made  proper  allowances  for  deficiencies  and 
excesses.  Twelve  moons  fell  short  of  the  true  year,  thirteen 
exceeded  it;  365  days  were  not  enough,  366  were  too  many. 
To  prevent  the  confusion  resulting  from  these  errors.  Julius 
Caesar  reformed  the  calendar  by  making  the  year  consist  of 
365  days,  6  hours  (which  is  hence  called  a  Julian  year),  and 
made  every  fourth  year  consist  of  366  days.  This  method  of 
reckoning  is  called  Old  Style. 

But  as  this  made  the  year  somewhat  too  long,  and  the  error 
in  1582  amounted  to  ten  days,  Pope  Gregory  XIII.,  in  order 
to  bring  the  vernal  equinox  back  to  the  2ist  of  March  again, 
ordered  ten  days  to  be  struck  out  of  that  year,  calling  the  next 


ASTRONOMY.  8 1 

day  after  the  4th  of  October  the  I5th;  and,  to  prevent  similar 
confusion  in  the  future,  he  decreed  that  three  leap-years  should 
be  omitted  in  the  course  of  every  four  hundred  years.  This 
way  of  reckoning  time  is  called  New  Style.  It  was  immedi- 
ately adopted  by  most  of  the  European  nations,  but  was  not 
accepted  by  the  English  until  the  year  1752.  The  error  then 
amounted  to  eleven  days,  which  were  taken  from  the  month  of 
September  by  calling  the  3d  of  that  month  the  I4th.  The  Old 
Style  is  still  retained  by  Russia. 

According  to  the  Gregorian  calendar,  every  year  whose  num- 
ber is  divisible  by  four  \*  a  leap-year,  except,  that,  in  the  case 
of  the  years  whose  numbers  are  exact  hundreds,  those  only  are 
leap-years  which  arc  divisible  by  four  after  cutting  off  the  last 
two  figures.  Thus  the  years  1600,  2000,  2400,  etc.,  are  leap- 
years;  1700,  1800,  1900,  2100,  2200,  etc.,  are  not.  The  error 
will  not  amount  to  a  clay  in  over  three  thousand  years. 

84.  The  Dominical  Letter.  —  The  dominical  letter  for  any 
year  is  that  which  we  often  see  placed  against  Sunday  in  the 
almanacs,  and  is  always  one  of  the  first  seven  in  the  alphabet. 
Since  a  common  year  consists  of  365  days,  if  this  number  is 
divided  by  seven  (the  number  of  days  in  a  week),  there  will  be 
a  remainder  of  one  :  hence  a  year  commonly  begins  one  day 
later  in  the  week  than  the  preceding  one  did.  If  a  year  of 
365  days  begins  on  Sunday,  the  next  will  begin  on  Monday; 
if  it  begins  on  Thursday,  the  next  will  begin  on  Friday;  and 
so  on.  If  Sunday  falls  on  the  ist  of  January,  the  first  letter 
of  the  alphabet,  or  A,  is  the  dominical  letter.  If  Sunday  falls 
on  the  7th  of  January  (as  it  will  the  next  year,  unless  the  first 
is  leap-year),  the  seventh  letter,  G,  is  the  dominical  letter.  If 
Sunday  falls  on  the  6th  of  January  (as  it  will  the  third  year, 
unless  the  first  or  second  is  leap-year),  the  sixth  letter,  F9  will 
be  the  dominical  letter.  Thus,  if  there  were  no  leap-years,  the 
dominical  letters  would  regularly  follow  a  retrograde  order, 
G,  F,  E,  D,  C,  B,  A. 

But  leap-years  have  366  days,  which,  divided  by  seven, 
leaves  two  remainder :  hence  the  years  following  leap-years  will 
begin  two  clays  later  in  the  week  than  the  leap-years  did.  To 
prevent  the  interruption  which  would  hence  occur  in  the  order 
of  the  dominical  letters,  leap-years  have  two  dominical  letters, 


82 


ASTRONOMY. 


one  indicating  Sunday  till  the  2Qth  of  February,  and  the  other 
for  the  rest  of  the  year. 

By  Table  L  below,  the  dominical  letter  for  any  year  (New 


TABLE  I. 

TABLE  II. 

Centuries. 

A 

:B|C 

[p 

E 

F    G 

100    200'  300 
500   600!  700 

400 

800 

i 

*    3 

i   4 

5 

6     7 

900  looo'i  100^200 

Jan.  31. 

8 

9  !  10  j  1  1 

12 

13   H 

Years  less  than 
One  Hundred. 

1300  1400  1500 
11700  1800  1900 

2IOO  22OO  23OO 

2500  2600  2700 

1000 
2OOO 
2400 
2800 

Oct.  31. 

15 

22 
29 

;  16  1  17  18 

|23J24|25 

26 

20 
27 

21 
28 

29OO  3000.3100 

32OO 

" 

I 

2 

~ 

33003400|  3500 
3700  3800  3900 

3600 
4000 

Feb.  28-29. 

5 

6     7 

8 

9 

IO 

II 

C     E 

G 

BA 

March  31.      12 

13;  14 

15 

16 

17 

18 

I 
2 

29 

57 

II 

B 
A 

D 
C 

F 
E 

G 
F 

Nov.  30.        26 

27:28 

29 

23 
30 

24 
31 

25 

3 

4 

I 

7 

3J: 

33 
34 
35 

g 

61 
62 

63 

% 

89 
90 
9i 

G 

IFE 

D 
C 
B 

B 
AG 
F 
E 
D 

D 
CB 

A 
f* 

F 

DC 

A 
G 

April  30. 
i  July  31 

2 

23 
3O 

3;  4 

10    II 

17  18 
24  25 
3i  '  . 

19 
26 

'3 

20 

27 

7 

14 

21 

28 

i 
8 
15 

22) 

29  1 

X 

-5  A 

hi 

Q2 

AG 

(  '  H 

H  1) 

FE 

; 

9 

10 
ii 

12 

39 
40 

65 

66 

67 
68 

93~ 

94 
95 
96 

E 
D 

CB 

"A" 

G 
F 
ED 

B 
A 
GF 

C 
B 
AG 

Aug.  31. 

.  .      ..  i     I 

61    71   8 
13  i  Hi  15 

2O    21    22 

27  !  28  !  29 

2 

16 
23 
30 

3 

10 

17 
24 
31 

4 
ii 

18 
25 

5 

12 

19 
26 

•  •  i 

U 

41 

6y 

y1/ 

A 

C 

E 

F 

i 

2 

H 
15 
16 

42 
43 
44 

45 

70 

72 
73 

98 

99 

F 
ED 
C 

B 
A 
GF 

E 

D 
C 
BA 

G 

E 
D 
CB 

Sept.  30. 
Dec.  31. 

31    4;     5 
10    II     12 

17   18  19 

24  1  25  26 

6 

13 

20 

27 

7 
14 

21 

28 

8 
15 

22 
29 

9 
16 

2o' 

18 

40 

71 

H 

1) 

F 

G 

"2  T 

19 

.  . 

A 

C 

E 

F 

— 



6 

20 

48 

76 

GF 

BA 

DC 

ED 

7 

8      Q 

10 

4 
ii 

12 

21 

49 

77 

E 

G 

B 

C 

May  31. 

H 

15    16 

17 

18 

19 

20 

22 

50 

7* 

D 

F 

A 

B 

-->    T 

22    23 

24 

2S 

26 

27 

23 

24 

52 

B 

.  . 

C 
BA 

E 
DC 

G 
FE 

A 

GF 

28 

29    30 

11 

T 



— 

3 

53 
54 

81 

82 

•• 

G  !  B 

F  |  A 

D 
C 

E 
D 

June  30. 

4 
ii 

5     6 

12     13 

7 
H 

8 
IS 

9 
16 

10 

17 

27 
28 

55 
56 

84 

E    G 
DCFE 

B    C 
AGIBA 

2C 

19    20 
26    27 

21 

28 

22 
29 

23(24 
30  .. 

Style)  for  four  thousand  years  from  the  beginning  of  the  Chris- 
tian Era  may  be  found ;  and  it  will  be  readily  seen  how  the 


ASTRONOMY.  83 

Table  could  be  extended  indefinitely  by  continuing  the  centu- 
ries at  the  top  in  the  same  order. 

To  find  the  dominical  letter  by  this  table,  look  for  the  hun- 
dreds of  years  at  the  top,  and  for  the  years  below  a  hundred, 
at  the  left  hand. 

Thus  the  letter  for  1882  will  be  opposite  the  number  82. 
and  in  the  column  having  1800  at  the  top;  that  is,  it  will  be  A. 
In  the  same  way,  the  letters  for  1884,  which  is  a  leap-year,  will 
be  found  to  be  FE. 

Having  the  dominical  letter  of  any  year.  Table  II.  shows 
what  days  of  every  month  of  the  year  will  be  Sundays. 

To  find  the  Sundays  of  any  month  in  the  year  by  this  table, 
look  in  the  column,  under  the  dominical  letter,  opposite  the 
name  of  the  month  given  at  the  left. 

From  the  Sundays  the  date  of  any  other  day  of  the  week 
can  be  readily  found. 

Thus,  if  we  wish  to  know  on  what  day  of  the  week  Christ- 
mas falls  in  1889,  we  look  opposite  December,  under  the  letter 
F  (which  we  have  found  to  be  the  dominical  letter  for  the  year), 
and  find  that  the  22d  of  the  month  is  a  Sunday;  the  25th,  or 
Christmas,  will  then  be  Wednesday. 

In  the  same  way  we  may  find  the  day  of  the  week  corre- 
sponding to  any  date  (New  Style)  in  history.  For  instance,  the 
1 7th  of  June,  1775,  the  day  of  the  fight  at  Bunker  Hill,  is  found 
to  have  been  a  Saturday. 

These  two  tables  then  serve  as  a  perpetual  almanac. 

WEIGHT  OF  THK  EARTH  AND  PRECESSION. 

85.  The  Weight  of  the  Earth.  —  There  are  several  methods 
of  ascertaining  the  weight  and  mass  of  the  earth.  The  sim- 
plest, and  perhaps  the  most  trustworthy  method  is  to  compare 
the  pull  of  the  earth  upon  a  ball  of  lead  with  that  of  a  known 
mass  of  lead  upon  it.  The  pull  of  a  known  mass  of  lead  upon 
the  ball  may  be  measured  by  means  of  a  torsion  balance.  One 
form  of  the  balance  employed  for  this  purpose  is  shown  in 
Figs.  98  and  99.  Two  small  balls  of  lead,  b  and  b,  are  fastened 
to  the  ends  of  a  light  rod  e,  which  is  suspended  from  the  point 
F  by  means  of  the  thread  FE.  Two  large  balls  of  lead,  IV 
and  W,  are  placed  on  a  turn-table,  so  that  one  of  them  shall 


84 


ASTRONOMY. 


be  just  in  front  of  one  of  the  small  balls,  and  the  other  just 
behind  the  other  small  ball.  The  pull  of  the  large  balls  turns 
the  rod  around  a  little  so  as  to  bring  the  small  balls  nearer  the 
large  ones.  The  small  balls  move  towards  the  large  ones  till 


•w 


TV- 


they  are  stopped  by  the  torsion  of  the  thread,  which  is  then 
equal  to  the  pullof  the  large  balls.  The  deflection  of  the  rod 
is  carefully  measured.  The  table  is  then  turned  into  the  posi- 
tion indicated  by  the  dotted  lines  in  Fig.  99,  so  as  to  reverse 
the  position  of  the  large  balls  with  reference  to  the  small  ones. 


Fig.  99. 

The  rod  is  now  deflected  in  the  opposite  direction,  and  the 
amount  of  deflection  is  again  carefully  measured.  The  second 
measurement  is  made  as  a  check  upon  the  accuracy  of  the  first. 
The  force  required  to  twist  the  thread  as  much  as  it  was 


ASTRONOMY.  85 

twisted  by  the  deflection  of  the  rod  is  ascertained  by  measure- 
ment. This  gives  the  pull  of  the  two  large  balls  upon  the  two 
small  ones.  We  next  calculate  what  this  pull  would  be  were 
the  balls  as  far  apart  as  the  small  balls  are  from  the  centre  of 
the  earth.  We  can  then  form  the  following  proportion:  the 
pull  of  the  large  balls  upon  the  small  ones  is  to  the  pull  of  the 
earth  upon  the  small  ones  as  the  mass  of  the  large  balls  is  to 
the  mass  of  the  earth,  or  as  the  weight  of  the  large  balls  is 
to  the  weight  of  the  earth.  Of  course,  the  pull  of  the  earth 
upon  the  small  balls  is  the  weight  of  the  small  balls.  In  this 
way  it  has  been  ascertained  that  the  mass  of  the  earth  is  about 
5.6  times  that  of  a  globe  of  water  of  the  same  size.  In  other 
words,  the  mean  density  of  the  earth  is  about  5.6. 

The  weight  of  the  earth  in  pounds  may  be  found  by  multi- 
plying the  number  of  cubic  feet  in  it  by  62^  (the  weight,  in 
pounds,  of  one  cubic  foot  of  water),  and  this  product  by  5.6. 

86.  Cause  of  Precession.  —  We  have  seen  that  the  earth  is 


Fig.  100. 

flattened  at  the  poles :  in  other  words,  the  earth  has  the  form 
of  a  sphere,  with  a  protuberant  ring  around  its  equator.  This 
equatorial  ring  is  inclined  to  the  plane  of  the  ecliptic  at  an  angle 
of  about  23-^°.  In  Fig.  100  this  ring  is  represented  as  detached 
from  the  enclosed  sphere.  6"  represents  the  sun,  and  Sc  the 
ecliptic.  As  the  point  A  of  the  ring  is  nearer  the  sun  than  the 
point  B  is,  the  sun's  pull  upon  A  is  greater  than  upon  B  : 
hence  the  sun  tends  to  pull  the  ring  over  into  the  plane  of  the 
ecliptic :  but  the  rotation  of  the  earth  tends  to  keep  the  ring  in 
the  same  plane.  The  struggle  between  these  two  tendencies 
causes  the  earth,  to  which  the  ring  is  attached,  to  wabble  like 
a  spinning-top,  whose  rotation  tends  to  keep  it  erect,  while 
gravity  tends  to  pull  it  over.  The  handle  of  the  top  has  a 
gyratory  motion,  which  causes  it  to  describe  a  curve.  The  axis 
of  the  heavens  corresponds  to  the  handle  of  the  top. 


86  ASTRONOMY. 

II.    THE   MOON. 
DISTANCE,  SIZE,  AND  MOTIONS. 

87.  The  Distance  of  the  Moon.  —  The  moon  is  the  near- 
est of  the  heavenly  bodies.  Its  distance  from  the  centre  of 
the  earth  is  only  about  sixty  times  the  radius  of  the  earth, 
or,  in  round  numbers,  two  hundred  and  forty  thousand  miles. 

The  ordinary  method  of  finding  the  distance  of  one  of  the 
nearer  heavenly  bodies  is  first  to  ascertain  its  horizontal  par- 
allax. This  enables  us  to  form  a  right-angled  triangle,  the 
lengths  of  whose  sides  are  easily  computed,  and  the  length  of 
whose  hypothenuse  is  the  distance  of  the  body  from  the  centre 
of  the  earth.  • 

Horizontal  parallax  has  already  been  defined  (32)  as  the  dis- 
placement of  a  heavenly 
body  when  on  the  horizon, 
caused  by  its  being  seen 
from  the  surface,  instead  of 

the    centre,   of    the    earth. 

lg>  IOX-  -ri  •      A-     i'  ,.    •      A 

i  his    displacement    is    due 

to  the  fact  that  the  body  is  seen  in  a  different  direction  from 
the  surface  of  the  earth  from  that  in  which  it  would  be  seen 
from  the  centre.  Horizontal  parallax  might  be  defined  as  the 
difference  in  the  directions  in  which  a  body  on  the  horizon 
would  be  seen  from  the  surface  and  from  the  centre  of  the 
earth.  Thus,  in  Fig.  101,  C  is  the  centre  of  the  earth,  A  a 
point  on  the  surface,  and  B  a  body  on  the  horizon  of  A.  A  B 
is  the  direction  in  which  the  body  would  be  seen  from  A,  and 
CB  the  direction  in  which  it  would  be  seen  from  C.  The  dif- 
ference of  these  directions,  or  the  angle  ABC,  is  the  parallax 
of  the  body. 

The  triangle  BA  C  is  right-angled  at  A;  the  side  A  C  is  the 
radius  of  the  earth,  and  the  hypothenuse  is  the  distance  of  the 
body  from  the  centre  of  the  earth.  When  the  parallax  A  BC 
is  known,  the  length  of  CB  can  easily  by  found  by  trigono- 
metrical computation. 

We  have  seen  (32)  that  the  parallax  of   a  heavenly  body 


ASTRONOMY.  87 

grows  less  and  less  as  the  body  passes  from  the  horizon 
towards  the  zenith.  The  parallax  of  a  body  and  its  altitude 
are.  however,  so  related,  that,  when  we  know  the  parallax  at 
any  altitude,  we  can  readily  compute  the  horizontal  parallax. 

The  usual  method  of  finding  the  parallax  of  one  of  the 
nearer  heavenly  bodies  is  first  to  find  its  parallax  when  on  the 
meridian,  as  seen  from  two  places  on  the  earth  which  differ 
considerably  in  latitude :  then  to  calculate  what  would  be  the 
parallax  of  the  body  as  seen  from  one  of  these  places  and  the 
centre  of  the  earth :  and  then  finally  to  calculate  what  would 
be  the  parallax  were  the  body  on  the  horizon. 

Thus,  we  should  ascertain  the  parallax  of  the  body  B  (Fig. 
102)  as  seen  from  A  and  D,  or  the  angle  ABD.  We  should 
then  calculate  its  parallax  as  seen  from  A  and  C,  or  the  angle 
ABC.  Finally  we  should  calculate  what  its  parallax  would  be 
were  the  body  on  the  horizon,  or  the  angle  A  B'  C. 

The  simplest  method  of 
finding  the  parallax  of  a 
body  B  (Fig.  102)  as  seen 
from  the  two  points  A  and 
D  is  to  compare  .its  direc- 
tion at  each  point  with  that 
of  the  same  fixed  star  near 
the  body.  The  star  is  so 
distant,  that  it  will  be  seen 
in  the  same  direction  from 
both  points  :  hence,  if  the 
direction  of  the  body  differs  from  that  of  the  star  2°  as  seen 
from  one  point,  and  2°  6'  as  seen  from  the  other  point,  the  two 
lines  AB  and  DB  must  differ  in  direction  by  6';  in  other 
words,  the  angle  A  B  D  would  be  6'. 

The  method  just  described  is  the  usual  method  of  finding 
the  parallax  of  the  moon. 

88.  The  Apparent  Size  of  the  Moon. — The  apparent  size 
of  a  body  is  the  visual  angle  subtended  by  it :  that  is,  the 
angle  formed  by  two  lines  drawn  from  the  eye  to  two  oppo- 
site points  on  the  outline  of  the  body.  The  apparent  size 
of  a  body  depends  upon  both  its  magnitude  and  its  distance. 


88 


ASTRONOMY. 


The  apparent  size,  or  angular  diameter,  of  the  moon  is 
about  thirty-one  minutes.  This  is  ascertained  by  means  of 
the  wire  micrometer  already  described  (19).  The  instru- 
ment is  adjusted  so  that  its  longitudinal  wire  shall  pass 
through  the  centre  of  the  moon,  and  its  transverse  wires 
shall  be  tangent  to  the  limbs  of  the 
moon  on  each  side,  at  the  point 
where  they  are  cut  by  the  longitu- 
dinal wire.  The  micrometer  screw 
is  then  turned  till  the  wires  are 
brought  together.  The  number  of 
turns  of  the  screw  needed  to  accom- 
plish this  will  indicate  the  arc  be- 
tween the  wires,  or  the  angular 
diameter  of  the  moon. 

In  order  to  be  certain  that  the  longitudinal  wire  shall  pass 
through  the  centre  of  the  moon,  it  is  best  to  take  the  moon 
when  its  disc  is  in  the  form  of  a  crescent,  and  to  place  the 


Fig.  104. 

longitudinal  wire  against  the  points,  of  cusps,  of  the  crescent, 
as  shown  in  Fig.  103. 

89.  The.  Real  Size  of  tlie  Moon.  —  The  real  diameter  of 
the  moon  is  a  little  over  one-fourth  of  that  of  the  earth,  or 
a  little  more  than  two  thousand  miles.  The  comparative 
sizes  of  the  earth  and  moon  are  shown  in  Fig.  104. 


ASTRONOMY. 


89 


The  distance  and  apparent  size  of  the  moon  being  known, 
her  real  diameter  is  found  by  means  of  a  triangle  formed  as 
shown  in  Fig.  105.  C  represents  the  centre  of  the  moon,  CB 
the  distance  of  the  moon  from  the  earth,  and  C  A  the  radius  of 
the  moon.  B  A  C  is  a  triangle,  right-angled  at  A.  The  angle 
ABC  is  half  the  apparent 
diameter  of  the  moon. 
With  the  angles  A  and  B, 
and  the  side  CB  known,  it 
is  easy  to  find  the  length 
of  AC  by  trigonometrical 
computation.  Twice  A  C  will  be  the  diameter  of  the  moon. 

The  volume  of  the  moon  is  about  one-fiftieth  of  that  of 
the  earth. 

90.  Apparent  Size  of  the  Moon  on  the  Horizon  and  in 
the  Zenith.  —  The  moon  is  nearly  four  thousand  miles  far- 
ther from  the  observer  when  she  is  on  the  horizon  than 
when  she  is  in  the  zenith.  This  is  evident  from  Fig.  106. 
C  is  the  centre  of  the  earth,  M  the  moon  on  the  hori- 
zon, M'  the  moon  in  the 
zenith,  and  O  the  point  of 
observation.  OM  is  the 
distance  of  the  moon  when 
she  is  on  the  horizon,  and 
OM'  the  distance  of  the 
moon  from  the  observer 
when  she  is  in  the  zenith. 
CM  is  equal  to  CM',  and 
OM  is  about  the  length  of  CM;  but  CM'  is  about  four 
thousand  miles  shorter  than  CM' :  hence  O  M'  is  about 
four  thousand  miles  shorter  than  O  M. 

Notwithstanding  the  moon  is  much  nearer  when  at  the 
zenith  than  at  the  horizon,  it  seems  to  us  much  larger  at  the 
horizon. 

This  is  a  pure  illusion,  as  we  become  convinced  when  we 
measure  the  disc  with  accurate  instruments,  so  as  to  make  the 


90  ASTRONOMY. 

result  independent  of  our  ordinary  way  of  judging.  When  the 
moon  is  near  the  horizon,  it  seems  placed  beyond  all  the  objects 
on  the  surface  of  the  earth  in  that  direction,  and  therefore  far- 
ther off  than  at  the  zenith,  where  no  intervening  objects  enable 
us  to  judge  of  its  distance.  In  any  case,  an  object  which  keeps 
the  same  apparent  magnitude  seems  to  us,  through  the  instinc- 
tive habits  of  the  eye,  the  larger  in  proportion  as  we  judge  it 
to  be  more  distant. 

91.  The  Apparent  Size  of  the  Moon  increased  by  Irradia- 
tion. —  In  the  case  of  the  moon,  the  word  apparent  means  much 
more  than  it  does  in  the  case  of  other  celestial  bodies.  Indeed, 
its  brightness  causes  our  eyes  to  play  us  false.  As  is  well 
known,  the  crescent  of  the  new  moon  seems  part  of  a  much 


Fig.  107. 

larger  sphere  than  that  which  it  has  been  said,  time  out  of  mind, 
to  "  hold  in  its  arms."  The  bright  portion  of  the  moon  as  seen 
with  our  measuring  instruments,  as  well  as  when  seen  with  the 
naked  eye,  covers  a  larger  space  in  the  field  of  the  telescope 
than  it  would  if  it  were  not  so  bright.  This  effect  of  irradia- 
tion, as  it  is  called,  must  be  allowed  for  in  exact  measurements 
of  the  diameter  of  the  moon. 

92.  Apparent  Size  of  the  Moon    in  Different  Parts    of 
her  Orbit.  —  Owing  to  the  eccentricity  of  the  moon's  orbit, 
her  distance  from  the  earth  varies  somewhat  from  time  to 
time.     This  variation   causes  a  corresponding  variation  in 
her  apparent  size,  which  is  illustrated  in  Fig.  107. 

93.  The  Mass  of  the  Moon.  — The  moon  is  considerably 


ASTRONOMY.  Q! 

less  dense  than  the  earth,  its  mass  being  only  about  one- 
eightieth  of  that  of  the  earth ;  that  is,  while  it  would  take 
only  about  fifty  moons  to  make  the  bulk  of  the  earth,  it 
would  take  about  eighty  to  make  the  mass  of  the  earth. 

One  method  of  finding  the  mass  of  the  moon  is  to  compare 
her  effect  in  producing  the  tides  with  that  of  the  sun.  We 
first  calculate  what  would  be  the  moon's  effect  in  producing  the 
tides,  were  she  as  far  off  as  the  sun.  We  then  form  the  follow- 
ing proportion  :  as  the  sun's  effect  in  producing  the  tides  is  to 
the  moon;s  effect  at  the  same  distance,  so  is  the  mass  of  the 
sun  to  the  mass  of  the  moon. 

The  method  of  finding  the  mass  of  the  sun  will  be  given 
farther  on. 

94.  The   Orbital  Motion    of  tJie   Moon.  —  If  we  watch 
the  moon  from  night  to  night,  we  see  that  she  moves  east- 
ward quite  rapidly  among  the  stars.     When  the  new  moon 
is  first  visible,  it  appears  near  the  horizon   in  the  west,  just 
after  sunset.     A  week  later  the  moon  will  be  on  the  meridian 
at  the  same  hour,  and  about  a  week  later  still  on  the  eastern 
horizon.     The  moon  completes  the  circuit  of  the  heavens 
in  a  period  of  about  thirty  days,  moving  eastward  at  the 
rate  of  about  twelve  degrees  a  day.     This  eastward  motion 
of  the  moon  is  due  to  the  fact  that  she  is  revolving  around 
the  earth  from  west  to  east. 

95.  The  Aspects  of  the  Moon.  —  As  the   moon  revolves 
around  the  earth,  she  comes   into   different   positions  with 
reference  to  the  earth  and  sun.     These  different  positions  of 
the  moon  are  called  the  aspects  of  the  moon.     The  four 
chief  aspects  of  the  moon  are  shown  in  Fig.  108.     When 
the  moon  is  at  M,  she  appears  in  the  opposite  part  of  the 
heavens  to  the  sun,  and  is  said  to  be  in  opposition ;  when  at 
M'  and  at  M'" ,  she  appears  ninety  degrees  away  from  the 
sun,  and  is   said   to   be   in   quadrature ;   when  at  M",  she 
appears  in  the  same  part  of  the  heavens  as  the  sun,  and  is 
said  to  be  in  conjunction. 


92 


ASTRONOMY. 


96.  The  Sidereal  and  Synodical  Periods  of  the  Moon. — 
The  sidereal  period  of  the  moon  is  the  time  it  takes  her  to 
pass  around  from  a  star  to  that  star  again,  or  the  time  it 

takes  her  to  make 
a  complete  revolution 
around  the  earth. 
This  is  a  period 
of  about  twenty-seven 
days  and  a  third.  It 
is  sometimes  called 
the  sidereal  month-. 

The  sy nodical  peri- 
od of  the  moon  is  the 
time  that  it  takes  the 
moon  to  pass  from 

one  aspect  around  to  the  same  aspect  again.  This  is  a 
period  of  about  twenty-nine  days  and  a  half,  and  it  is  some- 
times called  the  svnodicctl  month. 


Fig.     IOQ. 

The  reason  why  the  synodical  period  is  longer  than  the 
sidereal  period  will  appear  from  Fig.  109.  ,5"  represents  the 
position  of  the  sun,  E  that  of  the  earth,  and  the  small 


ASTRONOMY.  93 

circle  the  orbit  of  the  moon  around  the  earth.  The  arrow 
in  the  small  circle  represents  the  direction  the  moon  is 
revolving  around  the  earth,  and  the  arrow  in  the  arc  between 
E  and  E'  indicates  the  direction  of  the  earth's  motion  in 
its  orbit.  When  the  moon  is  at  Mi}  she  is  in  conjunction. 
As  the  moon  revolves  around  the  earth,  the  earth  moves 
forward  in  its  orbit.  When  the  moon  has  come  round  to 
/;/!,  so  that  m^m^  is  parallel  with  M«M^  she  will  have  made 
a  complete  or  sidereal  revolution  around  the  earth;  but 
she  will  not  be  in  conjunction  again  till  she  has  come  round 
to  M,  so  as  again  to  be  between  the  earth  and  sun.  That 


is  to  say,  the  moon  must  make  more  than  a  complete  revo- 
lution in  a  synodical  period. 

The  greater  length  of  the  synodical  period  is  also  evident 
from  Fig.  no.  /"represents  the  earth,  and  L  the  moon.  The 
arrows  indicate  the  direction  in  which  each  is  moving.  When 
the  earth  is  at  7",  and  the  moon  at  Z,  the  latter  is  in  conjunc- 
tion. When  the  earth  has  reached  7V,  and  the  moon  Z/,  the 
latter  has  made  a  sidereal  revolution ;  but  she  will  not  be 
in  conjunction  again  till  the  earth  has  reached  T" ',  and  the 
moon  L". 

97.  The  Phases  of  the  Moon.  —  When  the  new  moon 
appears  in  the  west,  it  has  the  form  of  a  crescent,  with  its 


94  ASTRONOMY. 

convex  side  towards  the  sun,  and  its  horns  towards  the  east, 


Fig    in. 

As   the    moon   advances   towards   quadrature,   the   crescent 
grows  thicker  and  thicker,  till  it  becomes  a  half-circle  at 


ASTRONOMY.  95 

first  quarter.  When  it  passes  quadrature,  it  begins  to  become 
convex  also  on  the  side  away  from  the  sun,  or  gibbous  in 
form.  As  it  approaches  opposition,  it  becomes  more  and 
more  nearly  circular,  until  at  opposition  it  is  a  full  circle. 
From  full  moon  to  last  quarter  it  is  again  gibbous,  and  at 
last  quarter  a  half-circle.  From  last  quarter  to  ne\v  moon 
it  is  again  crescent  ;  but  the  horns  of  the  crescent  are  now 
turned  towards  the  west.  The  successive  phases  of  the 
moon  are  shown  in  Fig.  1 1 1 . 

98.  Cause  of  the  Phases  of  the  Moon. — Take  a  globe, 
half  of  which  is  colored  white  and  the  other  half  black  in 
such  a  way  that  the  line  which  separates  the  white  and  black 
portions  shall  be  a  great  circle  which  passes  through  the 
poles  of  the  globe,  and  rotate  the  globe  slowly,  so  as  to 
bring  the  white  half  gradually  into  view.  When  the  white 
part  first  comes  into  view,  the  line  of  separation  between 
it  and  the  black  part,  which  we  may  call  the  terminator, 
appears  concave,  and  its  projection  on  a  plane  perpendicular 
to  the  line  of  vision  is  a  concave  line.  As  more  and  more 
of  the  white  portion  comes  into  view,  the  projection  of  the 
terminator  becomes  less  and  less  concave.  When  half  of 
the  white  portion  comes  into  view,  the  terminator  is  pro- 
jected as  a  straight  line.  When  more  than  half  of  the  white 
portion  comes  into  view,  the  terminator  begins  to  appear  as 
a  convex  line,  and  this  line  becomes  more  and  more  convex 
till  the  whole  of  the  white  half  comes  into  view,  when  the 
terminator  becomes  circular. 

The  moon  is  of  itself  a  dark,  opaque  globe ;  but  the  half 
that  is  towards  the  sun  is  always  bright,  as  shown  in  Fig.  112. 
This  bright  half  of  the  moon  corresponds  to  the  white  half 
of  the  globe  in  the  preceding  illustration.  As  the  moon 
revolves  around  the  earth,  different  portions  of  this  illumined 
half  are  turned  towards  the  earth.  At  new  moon,  when  the 
moon  is  in  conjunction,  the  bright  half  is  turned  entirely 
away  from  the  earth,  and  the  disc  of  the  moon  is  black  and 


g  ASTRONOMY. 

invisible.  Between  new  moon  and  first  quarter,  less  than 
half  of  the  illumined  side  is  turned  towards  the  earth,  and 
we  see  this  illumined  portion  projected  as  a  crescent.  At 
first  quarter,  just  half  of  the  illumined  side  is  turned  towards 
the  earth,  and  we  see  this  half  projected  as  a  half-circle. 


Fig.  112. 

Between  first  quarter  and  full,  more  than  half  of  the  illu- 
mined side  is  turned  towards  the  earth,  and  we  see  it  as 
gibbous.  At  full,  the  whole  of  the  illumined  side  is  turned 
towards  us,  and  we  see  it  as  a  full  circle.  From  full  to  new 
moon  again,  the  phases  occur  in  the  reverse  order. 


ASTRONOMY.  97 

99.  The  Form  of  the  Moon's   Orbit. — The  orbit  of  the 
moon  around  the  earth  is  an  ellipse  of  slight  eccentricity. 
The  form  of  this  ellipse  is  shown  in  Fig.  113.     C  is  the 
centre  of  the  ellipse,  and  E  the  position  of  the  earth  at  one 
of  its  foci.     The  eccentricity  of  the  ellipse  is  only  about 
one-eighteenth.     It  is  impossible  for  the  eye  to  distinguish 
such  an  ellipse  from  a  circle. 

100.  The  Inclination  of  the  Moon's  Orbit.  —  The  plane 


of  the  moon's  orbit  is  inclined  to  the  ecliptic  by  an  angle 
of  about  five  degrees.  The  two  points  where  the  moon's 
orbit  cuts  the  ecliptic  are  called  her  nodes.  The  moon's 
nodes  have  a  westward  motion  corresponding  to  that  of  the 
equinoxes,  but  much  more  rapid.  They  complete  the  cir- 
cuit of  the  ecliptic  in  about  nineteen  years. 

The  moon's  latitude  ranges  from   5°  north  to  5°  south; 
and  since,  owing  to  the  motion  of  her  nodes,  the  moon  is, 


98 


ASTRONOMY. 


during  a  period  of  nineteen  years,  5°  north  and  5°  south  of 
every  part  of  the  ecliptic,  her  declination  will  range  from 
23i°  -f  5°  =  28i°  north  to  23^  +  5°  =  28 J°  south. 

i  o  i .  The  Meridian  Altitude  of  the  Moon . — The  meridian 
altitude  of  any  body  is  its  altitude  when  on  the  meridian. 
In  our  latitude,  the  meridian  altitude  of  any  point  on  the 
equinoctial  is  forty-nine  degrees.  The  meridian  altitude  of 
the  summer  solstice  is  49°  4-  23-^°  =  72^°,  and  that  of  the 
winter  solstice  is  49°  —  23^°  =  25^°.  The  greatest  meridian 
altitude  of  the  moon  is  72^°  -f-  5°  =  77^°,  and  its  least 
meridian  altitude,  25!°  —  5°  =  2oJ°. 

When  the  moon's  meridian  altitude  is  greater  than  the 
^elevation  of  the  equinoctial,  it  is  said  to  run  high,  and  when 
less,  to  run  low.  The  full  moon  runs  high  when  the  sun  is 
south  of  the  equinoctial,  and  low  when  the  sun  is  north  of 
the  equinoctial.  This  is  because  the  full  moon  is  always  in 
the  opposite  part  of  the  heavens  to  the  sun. 

102.  Wet  and  Dry  Moon.  —  At  the  time  of  new  moon,  the 
cusps  of  the  crescent  sometimes  lie  in  a  line  which  is  nearly 
perpendicular  with  the  horizon,  and  sometimes  in  a  line  which 
is  nearly  parallel  with  the  horizon.  In  the  former  case  the 
moon  is  popularly  described  as  a  wet  moon,  and  in  the  latter 

case  as  a  dry  moon. 

The  great  circle 
which  passes  through 
the  centre  of  the  sun 
and  moon  will  pass 
through  the  centre  of 
the  crescent,  and  be 
perpendicular  to  the 
line  joining  the  cusps. 
Now  the  ecliptic  makes 
the  least  angle  with  the  horizon  when  the  vernal  equinox  is 
on  the  eastern  horizon  and  the  autumnal  equinox  is  on  the 
western.  In  our  latitude,  as  we  have  seen,  this  angle  is  25^°: 
hence  in  our  latitude,  if  the  moon  were  at  new  on  the  ecliptic 


Fig.  114. 


ASTRONOMY. 


99 


when  the  sun  is  at  the  autumnal  equinox,  as  shown  at  Ms 
(Fig.  114),  the  great  circle  passing  through  the  centre  of  the 
sun  and  moon  would  be  the  ecliptic,  and  at  New  York  would 
be  inclined  to  the  horizon  at  an  angle  of  25^°.  If  the  moon 
happened  to  be  5°  south  of  the  ecliptic  at  this  time,  as  at 
M±,  the  great  circle  pass- 
ing through  the  centre  of 
the  sun  and  moon  would 
make  an  angle  of  only 
20-^°  with  the  horizon. 
In  either  of  these  cases 
the  line  joining  the  cusps 
would  be  nearly  perpen- 
dicular to  the  horizon. 

If  the  moon  were  at 
new  on  the  ecliptic  when 
the  sun  is  near  the  vernal 
equinox,  as  shown  at  M\ 
(Fig.  115),  the  great  circle 

passing  through  the  centres  of  the  sun  and  moon  would  make 
an  angle  of  72^°  with  the  horizon  at  New  York ;  and  were  the 
moon  5°  north  of  the  ecliptic  at  that  time,  as  shown  at  M»,  this 
great  circle  would  make  an  angle  of  77^°  with  the  horizon.  In 
either  of  these  cases,  the -line  joining  the  cusps  would  be  nearly 
parallel  with  the  horizon. 

At  different  times,  the  line  joining  the  cusps  may  have  every 
possible  inclination  to  the  horizon  between  the  extreme  cases 
shown  in  Figs.  114  and  115. 

103.  Daily  Retardation  of  the  Moon's  Rising. — The 
moon  rises,  on  the  average,  about  fifty  minutes  later  each 
day.  This  is  owing  to  her  eastward  motion.  As  the  moon 
makes  a  complete  revolution  around  the  earth  in  about 
twenty-seven  days,  she  moves  eastward  at  the  rate  of  about 
thirteen  degrees  a  day,  or  about  twelve  degrees  a  day  faster 
than  the  sun.  Were  the  moon,  therefore,  on  the  horizon 
at  any  hour  to-day,  she  would  be  some  twelve  degrees  below 
the  horizon  at  the  same  hour  to-morrow.  Now,  as  the  hori- 


IOO  ASTRONOMY. 

zon  moves  at  the  rate  of  one  degree  in  four  minutes,  it 
would  take  it  some  fifty  minutes  to  come  up  to  the  moon  so 
as  to  bring  her  upon  the  horizon.  Hence  the  daily  retarda- 
tion of  the  moon's  rising  is  about  fifty  minutes ;  but  it 

varies  considerably  in  diifer- 
ent  parts  of  her  orbit. 

There  are  two  reasons  for 
this    variation     in     the-    daily 
~~N    retardation  :  — 

(i)  The  moon  moves  at  a 
varying  rate  in  her  orbit ;  her 
speed  being  greatest  at  perigee, 
and  least  at  apogee :  hence, 
other  things  being  equal,  the 
retardation  is  greatest  when 
the  moon  is  at  perigee,  and  least  when  she  is  at  apogee. 

(2)  The  moon  moves  at  a  varying  angle  to  the  horizon. 
The  moon  moves  nearly  in  the  plane  of  the  ecliptic,  and  of 
course  she  passes  both  equinoxes  every  lunation.  When  she 
is  near  the  autumnal  equinox,  her  path  makes  the  greatest 
angle  with  the  eastern  horizon,  and  when  she  is  near  the 
vernal  equinox,  the  least  angle :  E  . 

hence  the  moon  moves  away 
from  the  horizon  fastest  when 
she  is  near  the  autumnal  equi- 
nox, and  slowest  when  she  is  s 
near  the  vernal  equinox.  This 
will  be  evident  from  Figs.  116 
and  117.  In  each  figure,  SN 
represents  a  portion  of  the 
eastern  horizon,  and  EC,  E'  c', 
a  portion  of  the  ecliptic.  A  E, 
in  Fig.  1 1 6,  represents  the  autumnal  equinox,  and  ARM  the 
daily  motion  of  the  moon.  VR,  in  Fig.  117,  represents  the 
vernal  equinox,  and  VE  M'  the  motion  of  the  moon  for  one 
day.  In  the  first  case  this  motion  would  carry  the  moon  away 
from  the  horizon  the  distance  A  M,  and  in  the  second  case  the 


ASTRONOMY. '  /'  *  5-  "J   1GI' 

distance  A'M1.  Now,  it  is  evident  that  AM  is  greater  than 
y4 ' M' :  hence,  other  things  being  equal,  the  greatest  retardation 
of  the  moon's  rising  will  be  when  the  moon  is  near  the  autum- 
nal equinox,  and  the  least  retardation  when  the  moon  is  near 
the  vernal  equinox. 

The  least  retardation  at  New  York  is  twenty-three  minutes, 
and  the  greatest  an  hour  and  seventeen  minutes.  The  great- 
est and  least  retardations  vary  somewhat  from  month  to 
month  ;  since  they  depend  not  only  upon  the  position  of  the 
moon  in  her  orbit  with  reference  to  the  equinoxes,  but  also 


Fig.  118. 

upon  the  latitude  of  the  moon,  and  upon  her  nearness  to 
the  earth. 

The  direction  of  the  moon's  motion  with  reference  to 
the  ecliptic  is  shown  in  Fig.  118,  which  shows  the  moon's 
motion  for  one  day  in  July,  1876. 

104.  The  Harvest  Moon.  —  The  long  and  short  retarda- 
tions in  the  rising  of  the  moon,  though  they  occur  every 
month,  are  not  likely  to  attract  attention  unless  they  occur 
at  the  time  of  full  moon.  The  long  retardations  for  full 
moon  occur  when  the  moon  is  near  the  autumnal  equinox 
at  full.  As  the  full  moon  is  always  opposite  to  the  sun,  the 


.IO2  ASTRONOMY. 

sun  must  in  this  case  be  near  the  vernal  equinox :  hence 
the  long  retardations  for  full  moon  occur  in  the  spring,  the 
greatest  retardation  being  in  March. 

The  least  retardations  for  full  moon  occur  when  the  moon 
is  near  the  vernal  equinox  at  full :  the  sun  must  then  be 
near  the  autumnal  equinox.  Hence  the  least  retardations  for 
full  moon  occur  in  the  months  of  August,  September,  and 
October.  The  retardation  is,  of  course,  least  for  September ; 
and  the  full  moon  of  this  month  rises  night  after  night  less 
than  half  an  hour  later  than  the  previous  night.  The  full 
moon  of  September  is  called  the  "  Harvest  Moon,"  and  that 
of  October  the  "  Hunter's  Moon." 

105.  The  Rotation  of  the  Moon.  —  A   careful   examina- 
tion of  the  spots  on  the  disc  of  the  moon  reveals  the  fact 
that  she  always  presents  the   same   side  to  the  earth.     In 
order  to  do  this,  she  must  rotate  on  her  axis  while  making  a 
revolution  around  the  earth,  or  in  about  twenty-seven  days. 

1 06.  Librations  of  the  Moon.  —  The    moon    appears  to 
rock  slowly  to  and  fro,  so  as  to  allow  us  to  see  alternately  a 
little  farther  around  to  the  right  and  the  left,  or  above  and 
below,  than  we  otherwise  could.     This  apparent  rocking  of 
the  moon  is  called  libration.     The  moon  has  three  libra- 
tions  :  — 

1 i )  Libration  in  Latitude.  —  This  libration    enables    us 
to  see  alternately  a  little  way  around  on  the  northern  and 
southern  limbs  of  the  moon. 

This  libration  is  due  to  the  fact  that  the  axis  of  the  moon 
is  not  quite  perpendicular  to  the  plane  of  her  orbit.  The 
deviation  from  the  perpendicular  is  six  degrees  and  a  half.  As 
the  axis  of  the  moon,  like  that  of  the  earth,  maintains  the  same 
direction,  the  poles  of  the  moon  will  be  turned  alternately  six 
degrees  and  a  half  toward  and  from  the  earth. 

(2)  Libration  in  Longitude .  —*•  This  libration  enables  us- 
to  see  alternately  a  little  farther  around  on  the  eastern  and 
western  limbs  of  the  moon. 


ASTRONOMY. 


103 


It  is  due  to  the  fact  that  the  moon's  axial  motion  is  uniform, 
while  her  orbital  motion  is  not.  At  perigee  her  orbital  motion 
will  be  in  advance  of  her  axial  motion,  while  at  apogee  the 
axial  motion  will  be  in  advance  of  the  orbital.  In  Fig.  119, 
E  represents  the  earth.  M  the  moon,  the,  large  arrow  the 
direction  of  the  moon's  motion  in  her  orbit,  and  the  small 
arrow  the  direction  of  her  motion  of  rotation.  When  the 
moon  is  at  Af,  the  line  AB,  drawn  perpendicular  to  EM, 
represents  the  circle  which  divides  the  visible  from  the  invisible 
portion  of  the  moon.  While  the  moon  is  passing  from  M  to 
M',  the  moon  performs  less  than  a  quarter  of  a  rotation,  so 
that  AB  is  no  longer  perpendicular  to  EM'.  An  observer  on 
the  earth  can  now  see 
somewhat  beyond  A  on 
the  western  limb  of 
the  moon,  and  not  quite 
up  to  B  on  the  eastern 
limb.  While  the  moon 
is  passing  from  M'  to 
M",  her  axial  motion 
again  overtakes  her  or- 
bital motion,  so  that  the 
line  A  B  again  becomes 
perpendicular  to  the 
line  joining  the  centre 
of  the  moon  to  the 
centre  of  the  earth.  Exactly  the  same  side  is  now  turned 


Fig.  119. 


Exactly  the  same  side  is 
towards  the  earth  as  when  the  moon  was  at  M.  While  the 
moon  passes  from  M"  to  M"',  her  axial  motion  gets  in  advance 
of  her  orbital  motion,  so  that  A  B  is  again  inclined  to  the  line 
joining  the  centres  of  the  earth  and  moon.  A  portion  of  the 
eastern  limb  of  the  moon  beyond  B  is  now  brought  into  view 
to  the  earth,  and  a  portion  of  the  western  limb  at  A  is  carried 
out  of  view.  While  the  moon  is  passing  from  M'"  to  J/,  the 
orbital  motion  again  overtakes  the  axial  motion,  and  AB  is 
again  perpendicular  to  ME. 

(3)   Parallactic  Libration.  —  While   an   observer  at   the 
centre  of  the  earth  would  get  the  same  view  of  the  moon, 


IO4 


ASTRONOMY. 


whether  she  were  on  the  eastern  horizon,  in  the  zenith, 
or  on  the  western  horizon,  an  observer  on  the  surface  of 
the  earth  does  not  get  exactly  the  same  view  in  these 
three  cases.  When  the  moon  is  on  the  eastern  horizon, 
an  observer  on  the  surface  of  the  earth  would  see  a  little 
farther  around  on  the  western  limb  of  the  moon  than  when 
she  is  in  the  zenith,  and  not  quite  so  far  around  on  the  east- 
ern limb.  On  the  contrary,  when  the  moon  is  on  the 
western  horizon,  an  observer  on  the  surface  of  the  earth 
sees  a  little  farther  around  on  the  eastern  limb  of  the  moon 
than  when  she  is  in  the  zenith,  and  not  quite  so  far  around 
.on  her  western  limb. 


This  will  be  evident  from    Fig.    120.     E  is  the  centre  of 

the  earth,  and  O  a 
point  on  its  surface. 
A  B  is  aline  drawn 
through  the  centre 
of  the  moon,  per- 
pendicular to  a  line 
joining  the  centres 
of  the  moon  and 
the  earth.  This  line 
marks  off  the  part 
of  the  moon  turned 
towards  the  centre 
of  the  earth,  and  re- 
mains essentially  the  same  during  the  day.  CD  is  a  line  drawn 
through  the  centre  of  the  moon  perpendicular  to  a  line  joining 
the  centre  of  the  moon  and  the  point  of  observation.  This 
line  marks  off  the  part  of  the  moon  turned  towards  O.  When 
the  moon  is  in  the  zenith,  CD  coincides  with  A  B;  but,  when 
the  moon  is  on  the  horizon,  CD  is  inclined  to  A  B.  When  the 
moon  is  on  the  eastern  horizon,  an  observer  at  O  sees  a  little 
beyond  B,  and  not  quite  to  A;  and,  when  she  is  on  the  western 
horizon,  he  sees  a  little  beyond  A,  and  not  quite  to  B.  B  is 
on  the  western  limb  of  the  moon,  and  A  on  her  eastern  limb. 
Since  this  libration  is  due  to  the  point  from  which  the  moon 


ASTRONOMY. 


105 


is  viewed,  it  is  called  parallactic  libration  ;  and,  since  it  occurs 
daily,  it  is  called  diurnal  libration. 

107.  Portion  of  the  Lunar  Surface  brought  into  View  by 
Libration.  —  The  area  brought  into  view  by  the  first  two  libra- 
tions  is  between  one-twelfth  and  one-thirteenth  of  the  whole 
lunar  surface,  or  nearly  one-sixth  of  the  hemisphere  of  the  moon 
which  is  turned  away  from  the  earth  when  the  moon  is  at  her 
state  of  mean  libration.  Of  course  a  precisely  equal  portion 


Fig.  121. 

of  the  hemisphere  turned  towards  us  during  mean  libration  is 
carried  out  of  view  by  the  lunar  librations. 

If  we  add  to  each  of  these  areas  a  fringe  about  one  degree 
wide,  due  to  the  diurnal  libration,  and  which  we  may  call  the 
parallactic  fringe,  we  shall  find  that  the  total  area  brought  into 
view  is  almost  exactly  one-eleventh  part  of  the  whole  surface 
of  the  moon.  A  similar  area  is  carried  out  of  view ;  so  that 
the  whole  region  thus  swayed  out  of  and  into  view  amounts 
to  two-elevenths  of  the  moon's  surface.  This  area  is  shown 
in  Fig.  121,  which  is  a  side  view  of  the  moon. 


io6 


ASTRONOMY. 


1 08.  The  Moons  Path  through  Space.  —  Were  the  earth 
stationary,  the  moon  would  describe  an  ellipse  around  it  similar 
to  that  of  Fig.  113;  but,  as  the  earth  moves  forward  in  her 


Fig.  122. 

orbit  at  the  same  time  that  the  moon  revolves  around  it,  the 
moon  is  made  to  describe  a  sinuous  path,  as  shown  by  the 
continuous  line  in  Fig.  122.  This  feature  of  the  moon's  path  is 


Fig.  123. 

greatly  exaggerated  in  the  upper  portion  of  the  diagram.  The 
form  of  her  path  is  given  with  a  greater  degree  of  accuracy  in 
the  lower  part  of  the  figure  (the  broken  line  represents  the  path. 


ASTRONOMY. 


JO/ 


of  the  earth) ;  but  even  here  there  is  considerable  exaggeration. 
The  complete  serpentine  path  of  the  moon  around  the  sun  is 
shown,  greatly  exaggerated,  in  Fig.  123,  the  broken  line  being 
the  path  of  the  earth. 

The  path  described  by  the  moon  through  space  is  much  the 
same  as  that  described  by  a  point  on  the  circumference  of  a 
wheel  which  is  rolled  over  another  wheel.  If  we  place  a  cir- 
cular disk  against  the  wall,  and  carefully  roll  along  its  edge 
another  circular  disk  (to  which  a  piece  of  lead  pencil  has  been 


Fig.  124. 

fastened  so  as  to  mark  upon  the  wall),  the  curve  described  will 
somewhat  resemble  that  described  by  the  moon.  This  curve 
is  called  an  epicycloid,  and  it  will  be  seen  that  at  every  point 
it  is  concave  towards  the  centre  of  the  larger  disk.  In  the 
same  way  the  moon's  orbit  is  at  every  point  concave  towards 
the  sun. 

The  exaggeration  of  the  sinuosity  in   Fig.  123  will  be  more 
evident  when  it  is  stated,  that,  on  the  scale  of  Fig.  124,  the 


1 08 


ASTRONOMY. 


whole  of  the  serpentine  curve  would  lie  within  the  breadth  of 
the  fine  circular  line  MM'. 

109.  The  Lunar  Day. — The  lunar  day  is  twenty-nine 
times  and  a  half  as  long  as  the  terrestrial  day.  Near  the 
moon's  equator  the  sun  shines  without  intermission  nearly 
fifteen  of  our  days,  and  is  absent  for  the  same  length  of 
time.  Consequently,  the  vicissitudes  of  temperature  to 
which  the  surface  is  exposed  must  be  very  great.  During 
the  long  lunar  night  the  temperature  of  a  body  on  the 
moon's  surface  would  probably  fall  lower  than  is  ever  known 
on  the  earth,  while  during  the  day  it  must  rise  higher  than 
anywhere  on  our  planet. 


Fig.  125. 

It  might  seem,  that,  since  the  moon  rotates  on  her  axis  in 
about  twenty-seven  days,  the  lunar  day  ought  to  be  twenty- 
seven  days  long,  instead  of  twenty-nine.  There  is,  however, 
a  solar,  as  well  as  a  sidereal,  day  at  the  moon,  as  on  the  earth ; 
and  the  solar  day  at  the  moon  is  longer  than  the  sidereal  day, 
for  the  same  reason  as  on  the  earth.  During  the  solar  day  the 
moon  must  make  both  a  synodical  rotation  and  a  synodical 
revolution.  This  will  be  evident  from  Fig.  125,  in  which  is 
shown  the  path  of  the  moon  during  one  complete  lunation. 
E,  E' ,  E" ,  etc.,  are  the  successive  positions  of  the  earth ;  and 
i,  2,  3,  4,  5,  the  successive  positions  of  the  moon.  The  small 
arrows  indicate  the  -direction  of  the  moon's  rotation.  The 
moon  is  full  at  I  and  5.  At  i,  A,  at  the  centre  of  the  moon's 


ASTRONOMY.  1 09 

disk,  will  have  the  sun,  which  lies  in  the  direction  A  S,  upon  the 
meridian.  Before  A  will  again  have  the  sun  on  the  meridian, 
the  moon  must  have  made  a  synodical  revolution ;  and,  as  will 
be  seen  by  the  dotted  lines,  she  must  have  made  more  than  a 
complete  rotation.  The  rotation  which  brings  the  point  A  into 
the  same  relation  to  the'  earth  and  sun  is  called  a  synodical 
rotation. 

It  will  also  be  evident  from  this  diagram  that  the  moon  must 
make  a  synodical  rotation  during  a  synodical  revolution,  in 
order  always  to  present  the  same  side  to  the  earth. 

no.  The  Earth  as  seen  from  the  Moon.  —  To  an  ob- 
server on  the  moon,  the  earth  would  be  an  immense  moon, 
going  through  the  same  phases  that  the  moon  does  to  us ; 
but,  instead  of  rising  and  setting,  it  would  only  oscillate  to 
and  fro  through  a  few  degrees.  On  the  other  side  of  the 
moon  it  would  never  be  seen  at  all.  The  peculiarities  of 
the  moon's  motions  which  cause  the  librations,  and  make  a 
spot  on  the  moon's  disk  seem  to  an  observer  on  the  earth 
to  oscillate  to  and  fro,  would  cause  the  earth  as  a  whole  to 
appear  to  a  lunar  observer  to  oscillate  to  and  fro  in  the 
heavens  in  a  similar  manner. 

It  is  a  well-known  fact,  that,  at  the  time  of  new  moon,  the 
dark  part  of  the  moon's  surface  is  partially  illumined,  so 
that  it  becomes  visible  to  the  naked  eye.  This  must  be  due 
to  the  light  reflected  to  the  moon  from  the  earth.  Since  at 
new  moon  the  moon  is  between  the  earth  and  sun,  it  follows, 
that,  when  it  is  new  moon  at  the  earth,  it  must  be  full  earth 
at  the  moon :  hence,  while  the  bright  crescent  is  enjoying 
full  sunlight,  the  dark  part  of  its  surface  is  enjoying  the 
light  of  the  full  earth.  Fig.  126  represents  the  full  earth  as 
seen  from  the  moon. 

THE  ATMOSPHERE  OF  THE  MOON. 

in.  The  Moon  has  no  Appreciable  Atmosphere.  —  There 
are  several  reasons  for  believing  that  the  moon  has  little  or 
no  atmosphere. 


I  IO  ASTRONOMY. 

( i )   Had  the  moon  an  -atmosphere,  it  would  be  indicated 


Fig.  126. 

at  the  time  of  a  solar  eclipse,  when  the  moon  passes  over 
the  disk  of  the  sun.     If  the  atmosphere  were  of  any  con- 


ASTRONOMY. 


Ill 


.  127. 


siderable  density,  it  would  absorb  a  part  of  the  sun's  rays, 
so  as  to  produce  a  dusky  border  in  front  of  the  moon's  disk, 
as  shown  in  Fig.  127.  In  reality  no  such  dusky  border  is 
ever  seen ;  but  the  limb  of  the  moon  appears  sharp,  and 
clearly  defined,  as  in 
Fig.  128. 

If  the  atmosphere 
were  not  dense  enough 
to  produce  this  dusky 
border,  its  refraction 
would  be  sufficient 
to  distort  the  deli- 
cate cusps  of  the 
sun's  crescent  in  the 
manner  shown  at 
the  top  of  Fig.  125  ; 
but  no  such  distortion  is  ever  observed.  The  cusps  always 
appear  clear  and  sharp,  as  shown  at  the  bottom  of  the  figure  : 
hence  it  would  seem  that  there  can  be  no  atmosphere  of 

appreciable  density  at  the 

moon. 

(2)   The  absence  of  an 

atmosphere  from  the  moon 

fflSKS^^^^^"-'  HI  I  *s  a^so  snown  ky  the  ab- 
sence of  twilight  and  of 
diffused  daylight. 

Upon  the  earth,  twilight 
continues  until  the  sun  is 
eighteen  degrees  below  the 
horizon  ;  that  is,  day  and 
night  are  separated  by  a 

belt  twelve  hundred  miles  in  breadth,  in  which  the  transition 
from  light  to  darkness  is  gradual.  We  have  seen  (66)  that 
this  twilight  results  from  the  refraction  and  reflection  of 
light  by  our  atmosphere ;  and,  if  the  moon  had  an  atmos- 


112 


ASTRONOMY. 


phere,  we  should  notice  a  similar  gradual  transition  from 
the  bright  to  the  dark  portions  of  her  surface.  Such,  how- 
ever, is  not  the  case.  The  boundary  between  the  light  and 
darkness,  though  irregular,  is  sharply  defined.  Close  to  this 
boundary  the  unillumined  portion  of  the  moon  appears  just 
as  dark  as  at  any  distance  from  it. 

The  shadows  on  the  moon  are  also  pitchy  black,  without 
a  trace  of  diffused  daylight. 

(3)  The  absence  of  an  atmosphere  is  also  proved  by  the 
absence  of  refraction  when  the  moon  passes  between  us  and 
the  stars.  Let  A  B  (Fig.  129)  represent  the  disk  of  the  moon, 
and  CD  an  atmosphere  supposed  to  surround  it.  Let  SA  E 
represent  a  straight  line  from  the  earth,  touching  the  moon  at 
A,  and  let  S  be  a  star  situated  in  the  direction  of  this  line.  If 
c  the  moon  had  no 

"  atmosphere,  this 

star  would  appear 
to  touch  the  edge 
of  the  moon  at 
A;  but,  if  the 

,       , 

moon  had  an  at- 
mosphere, a  star  behind  the  edge  of  the  moon,  at  Sf,  would 
be  visible  at  the  earth;  for  the  ray  S'  A  would  be  bent  by  the 
atmosphere  into  the  direction  A  E'.  So,  also,  on  the  opposite 
side  of  the  moon,  a  star  might  be  seen  at  the  earth,  although 
really  behind  the  edge  of  the  moon  :  hence,  if  the  moon  had  an 
atmosphere,  the  time  during  which  a  star  would  be  concealed 
by  the  moon  would  be  less  than  if  it  had  no  atmosphere,  and 
the  amount  of  this  effect  must  be  proportional  to  the  density 
of  the  atmosphere. 

The  moon,  in  her  orbital  course  across  the  heavens,  is  con- 
tinually passing  before,  or  occulting,  some  of  the  stars  that  so 
thickly  stud  her  apparent  path  ;  and  when  we  see  a  star  thus 
pass  behind  the  lunar  disk  on  one  side,  and  come  out  again  on 
the  other  side,  we  are  virtually  observing  the  setting  and  rising 
of  that  star  upon  the  moon.  The  moon's  apparent  diameter 
has  been  measured  over  and  over  again,  and  is  known  with 


Fig.  129. 


ASTRONOMY.  113 

great  accuracy ;  the  rate  of  her  motion  across  the  sky  is  also 
known  with  perfect  accuracy :  hence  it  is  easy  to  calculate  how 
long  the  moon  will  take  to  travel  across  a  part  of  the  sky 
exactly  equal  in  length  to  her  own  diameter.  Supposing,  then, 
that  we  observe  a  star  pass  behind  the  moon,  and  out  again,  it 
is  clear,  that,  if  there  is  no  atmosphere,  the  interval  of  time 
during  which  it  remains  occulted  ought  to  be  exactly  equal  to 
the  computed  time  which  the  moon  would  take  to  pass  over  the 
star.  If,  however,  from  the  existence  of  a  lunar  atmosphere, 
the  star  disappears  too  late,  and  re-appears  too  soon,  as  we 
have  seen  it  would,  these  two  intervals  will  not  agree ;  the  com- 
puted time  will  be  greater  than  the  observed  time,  and  the 
difference  will  represent  the  amount  of  refraction  the  star's 
light  has  sustained  or  suffered,  and  hence  the  extent  of  atmos- 
phere it  has  had  to  pass  through. 

Comparisons  of  these  two  intervals  of  time  have  been 
repeatedly  made,  the  most  extensive  being  executed  under  the 
direction  of  the  Astronomer  Royal  of  England, 'several  years 
ago,  and  based  upon  no  less  than  two  hundred  and  ninety- 
six  occultation  observations.  In  this  determination  the  meas- 
ured or  telescopic  diameter  of  the  moon  was  compared  with 
the  diameter  deduced  from  the  occultations ;  and  it  was  found 
that  the  telescopic  diameter  was  greater  than  the  occultation 
diameter  by  two  seconds  of  angular  measurement,  or  by  about 
a  thousandth  part  of  the  whole  diameter  of  the  moon.  This 
discrepancy  is  probably  due,  in  part  at  least,  to  irradiation  (91), 
which  augments  the  apparent  size  of  the  moon,  as  seen  in  the 
telescope  as  well  as  with  the  naked  eye ;  but,  if  the  whole  two 
seconds  were  caused  by  atmospheric  refraction,  this  would 
imply  a  horizontal  refraction  of  one  second,  which  is  only  one 
two-thousandth  of  the  earth's  horizontal  refraction.  It  is  pos- 
sible that  an  atmosphere  competent  to  produce  this  refraction 
would  not  make  itself  visible  in  any  other  way. 

But  an  atmosphere  two  thousand  times  rarer  than  our  air 
can  scarcely  be  regarded  as  an  atmosphere  at  all.  The  con- 
tents of  an  air-pump  receiver  can  seldom  be  rarefied  to  a 
greater  extent  than  to  about  a  thousandth  of  the  density  of  air 
at  the  earth's  surface;  and  the  lunar  atmosphere,  if  it  exists 
at  all,  is  thus  proved  to  be  twice  as  attenuated  as  what  we 
commonly  call  a  vacuum. 


ASTRONOMY. 


THE  SURFACE  OF  THE  MOON. 

112.  Dusky  Patches  on  the  Disk  of  the  Moon.  —  With 
the  naked  eye,  large  dusky  patches  are  seen  on  the  moon, 
in  which  popular  fancy  has  detected  a  resemblance  to  a 
human  face.  With  a  telescope  of  low  power,  these  dark 
patches  appear  as  smooth  as  water,  and  they  were  once 


Fig.  130. 

supposed  to  be  seas.  This  theory  was  the  origin  of  the 
name  mare  (Latin  for  sea),  which  is  still  applied  to  the 
larger  of  these  plains ;  but,  if  there  were  water  on  the  sur- 
face of  the  moon,  it  could  not  fail  to  manifest  its  presence 
by  its  vapor,  which  would  form  an  appreciable  atmosphere. 
Moreover,  with  a  high  telescopic  power,  these  plains  present 


ASTRONOMY. 


a  more  or  less  uneven  surface ;  and,  as  the  elevations  and 
depressions  are  found  to  be  permanent,  they  cannot,  of 
course,  belong  to  the  surface  of  water. 

The  chief  of  these  plains  are  shown  in  Fig.  130.  They  are 
Mare  Crisium,  Mare  Fcecunditatis,  Mare  Nectaris,  Mare  Tran- 
quillitatis,  Mare  Serenitatis,  Mare  Imbrium,  Mare  Frigoris, 
and  Oceanus  Procellarum.  All  these  plains  can  easily  be  rec- 
ognized on  the  surface  of 
the  full  moon  with  the  un- 
aided eye. 

113.  The  Terminator 
of  the  Moon.  —  The 
terminator  of  the  moon 
is  the  line  which  sepa- 
rates the  bright  and  dark 
portions  of  its  disk. 
When  viewed  with  a 
telescope  of  even  mod- 
erate power,  the  termi- 
nator is  seen  to  be  very 
irregular  and  uneven. 
Many  bright  points  are 
seen  just  outside  of  the 
terminator  in  the  dark 
portion  of  the  disk,  while 
all  along  in  the  neigh- 
borhood of  the  termi- 
nator are  bright  patches 
and  dense  shadows.  These  appearances  are  shown  in  Figs. 
131  and  132,  which  represent  the  moon  near  the  first  and 
last  quarters.  They  indicate  that  the  surface  of  the  moon 
is  very  rough  and  uneven. 

As  it  is  always  either  sunrise  or  sunset  along  the  termi- 
nator, the  bright  spots  outside  of  it  are  clearly  the  tops  of 
mountains,  which  catch  the  rays  of  the  sun  while  their  bases 


Fig.  131. 


u6 


ASTRONOMY. 


are  in  the  shade.  The  bright  patches  in  the  neighborhood 
of  the  terminator  are  the  sides  of  hills  and  mountains  which 
are  receiving  the  full  light  of  the  sun,  while  the  dense 
shadows  near  by  are  cast  by  these  elevations. 

114.  Height  of  the  Lunar  Mountains. — There  are  two 
methods  of  finding  the  height  of  lunar  mountains  :  — 


Fig.  132. 

(i)  We  may  measure  the  length  of  the  shadows,  and 
then  calculate  the  height  of  the  mountains  that  would  cast 
such  shadows  with  the  sun  at  the  required  height  above  the 
horizon. 

The  length  of  a  shadow  may  be  obtained  by  the  following 
method :  the  longitudinal  wire  of  the  micrometer  (19)  is  adjusted 
so  as  to  pass  through  the  shadow  whose  length  is  to  be  meas- 


ASTRONOMY.  I  I/ 

ured,  and  the  transverse  wires  are  placed  one  at  each  end  of 
the  shadow,  as  shown  in  Fig.  133.  The  micrometer  screw  is 
then  turned  til!  the  wires  are  brought  together,  so  as  to  ascer- 
tain the  length  of  the  arc  between  them.  We  may  then  form 
the  proportion :  the  number  of  seconds  in  the  semi-diameter 
of  the  moon  is  to  the  number  of  seconds  in  the  length  of  the 
shadow,  as  the  length  of  the  moon's  radius  in  miles  to  the 
length  of  the  shadow  in  miles. 


Fig.  133- 

The  height  of  the  sun  above  the  horizon  is  ascertained 
by  measuring  the  angular  distance  of  the  mountain  from  the 
terminator. 

(2)  We  may  measure  the  distance  of  a  bright  point  from 
the  terminator,  and  then  construct  a  right-angled  triangle, 
as  shown  in  Fig.  134.  A  solution  of  this  triangle  will  enable 
us  to  ascertain  the  height  of  the  mountain  whose  top  is  just 
catching  the  level  rays  of  the  sun. 

B  is  the  centre  of  the  moon,  M  the  top  of  the  mountain, 


Il8  ASTRONOMY. 

and  SA  M  a  ray  of  sunlight  which  just  grazes  the  terminator 
at  A,  and  then  strikes  the  top  of  the  mountain  at  M.  The 
triangle  BA Mis  right-angled  at  A.  B A  is  the  radius  of  the 
moon,  and  A  M  is  known  by  measurement;  BM,  the  hypothe- 
nuse,  may  then  be  found  by  computation.  BM  is  evidently 
equal  to  the  radius  of  the  moon  plus  the  height  of  the  moun- 
tain. 

By  one  or  the  other  of  these  methods,  the  heights  of 
the  lunar  mountains  have  been  found  with  a  great  degree 
of  accuracy.  It  is  claimed  that  the  heights  of  the  lunar 
mountains  are  more  accuratelv  known  than  those  of  the 


Fig.  134- 

mountains  on  the  earth.  Compared  with  the  size  of  the 
moon,  lunar  mountains  attain  a  greater  height  than  those 
on  the  earth. 

115.  General  Aspect  of  the  Lunar  Surface.  —  A  cursory 
examination  of  the  moon  with  a  low  power  is  sufficient  to 
show  the  prevalence  of  crater-like  inequalities  and  the  gen- 
eral tendency  to  circular  shape  which  is  apparent  in  nearly 
all  the  surface  markings  ;  for  even  the  large  "  seas  "  and 
the  smaller  patches  of  the  same  character  repeat  in  their 
outlines  the  round  form  of  the  craters.  It  is  along  the 
terminator  that  we  see  these  crater-like  spots  to  the  best 
advantage  ;  as  it  is  there  that  the  rising  or  setting  sun  casts 


ASTRONOMY.  IIQ 

long  shadows  over  the  lunar  landscape,  and  brings  eleva- 
tions into  bold  relief.  They  vary  greatly  in  size  ;  some  being 
so  large  as  to  bear  a  sensible  proportion  to  the  moon's 
diameter,  while  the  smallest  are  so  minute  as  to  need  the 
most  powerful  telescopes  and  the  finest  conditions  of  atmos- 
phere to  perceive  them. 

The  prevalence  of  ring-shaped  mountains  and  plains  will 


Fig.  135- 

be  evident  from  Fig.  135,  which  is  from  a  photograph  of  a 
model  of  the  moon  constructed  by  Nasmyth. 

This  same  feature  is  nearly  as  marked  in  Figs.  131  and 
132,  which  are  copies  of  Rutherfurd's  photographs  of  the 
moon. 

1 1 6.  Lunar  Craters. — The  smaller  saucer-shaped  forma- 
tions on  the  surface  of  the  moon  are  called  craters.  They 


I2O  ASTRONOMY. 

are  of  all  sizes,  from  a  mile  to  a  hundred  and  fifty  miles  in 
diameter ;  and  they  are  supposed  to  be  of  volcanic  origin. 
A  high  telescopic  power  shows  that  these  craters  vary  re- 
markably, not  only  in  size,  but  also  in  structure  and  arrange- 
ment. Some  are  considerably  elevated  above  the  surrounding 
surface,  others  are  basins  hollowed  out  of  that  surface,  and 
with  low  surrounding  ramparts ;  some  are  like  walled  plains, 
while  the  majority  have  their  lowest  depression  considerably 
below  the  surrounding  surface ;  some  are  isolated  upon  the 
plains,  others  are  thickly  crowded  together,  overlapping  and 
intruding  upon  each  other;  some  have  elevated  peaks  or 
cones  in  their  centres,  and  some  are  without  these  central 
cones,  while  others,  again,  contain  several  minute  craters 
instead ;  some  have  their  ramparts  whole  and  perfect,  others 
have  them  broken  or  deformed,  and  many  have  them 
divided  into  terraces,  especially  on  their  inner  sides. 

A  typical  lunar  crater  is  shown  in  Fig.  136. 

It  is  not  generally  believed  that  any  active  volcanoes  exist 
on  the  moon  at  the  present  time,  though  some  observers 
have  thought  they  discerned  indications  of  such  volcanoes. 

117.  Copernicus.  —  This  is  one  of  the  grandest  of  lunar 
craters  (Fig.  137).  Although  its  diameter  (forty-six  miles) 
is  exceeded  by  others,  yet,  taken  as  a  whole,  it  forms  one  of 
the  most  impressive  and  interesting  objects  of  its  class. 
Its  situation,  near  the  centre  of  the  lunar  disk,  renders  all 
its  wonderful  details  conspicuous,  as  well  as  those  of  objects 
immediately  surrounding  it.  Its  vast  rampart  rises  to  up- 
wards of  twelve  thousand  feet  above  the  level  of  the  plateau, 
nearly  in  the  centre  of  which  stands  a  magnificent  group  of 
cones,  three  of  which  attain  a  height  of  more  than  twenty- 
four  hundred  feet. 

Many  ridges,  or  spurs,  may  be  observed  leading  away  from 
the  outer  banks  of  the  great  rampart.  Around  the  crater, 
extending  to  a  distance  of  more  than  a  hundred  miles  on 
every  side,  there  is  a  complex  network  of  bright  streaks, 


ASTRONOMY.  121 

which    diverge    in   all   directions.     These    streaks    do    not 


appear  in  the  figure,  nor  are  they  seen  upon  the  moon, 


122 


ASTRONOMY. 


except  at  and  near  the  full  phase.     They  show  conspicu- 
ously, however,  by  their  united  lustre  on  the  full  moon. 

This  crater  is  seen  just  to  the  south-west  of  the  large 
dusky  plain  in  the  upper  part  of  Fig.  132.  This  plain  is 
Mare  Imbrium,  and  the  mountain-chain  seen  a  little  to  the 
right  of  Copernicus  is  named  the  Apennines.  Copernicus 


same 


is  also  seen  in    Fig 
range. 

Under  circumstances  specially  favorable,  myriads  of  com- 
paratively minute  but  perfectly  formed  craters  may  be  ob- 
served for  more  than  seventy  miles  on  all  sides  around 
Copernicus.  The  district  on  the  south-east  side  is  specially 
rich  in  these  thickly  scattered  craters,  which  we  have  reason 
to  suppose  stand  over  or  upon  the  bright  streaks. 


ASTRONOMY. 


123 


1 1 8.  Dark  Chasms.  —  Dark  cracks,  or  chasms,  have  been 
observed  on  various  parts  of  the  moon's  surface.  They 
sometimes  occur  singly,  and  sometimes  in  groups.  They  are 
often  seen  to  radiate  from  some  central  cone,  and  they 
appear  to  be  of  volcanic  origin.  They  have  been  called 
canals  and  rills. 


Fig.  138. 

One  of  the  most  remarkable  groups  of  these  chasms  is 
that  to  the  west  of  the  crater  named  Triesneker.  The 
crater  and  the  chasms  are  shown  in  Fig.  138.  Several  of 
these  great  cracks  obviously  diverge  from  a  small  crater  near 
the  west  bank  of  the  great  one,  and  they  subdivide  as  they 
extend  from  the  apparent  point  of  divergence,  while  they 
are  crossed  by  others.  These  cracks,  or  chasms,  are  nearly 


124 


ASTRONOMY. 


a  mile  broad  at  the  widest  part,  and,  after  extending  full  a 
hundred  miles,  taper  away  till  they  become  invisible. 

119.  Mountain-Ranges.  —  There  are  comparatively   few 
mountain-ranges  on  the  moon.     The  three  most  conspicuous 


are  those  which  partially  enclose  Mare  Imbrium ;  namely, 
the  Apennines  on  the  south,  and  the  Caucasus  and  the 
Alps  on  the  east  and  north-east.  The  Apennines  are  the 
most  extended  of  these,  having  a  length  of  about  four  hun- 


ASTRONOMY. 


125 


dred  and  fifty  miles.  They  rise  gradually,  from  a  compara- 
tively level  surface  towards  the  south-west,  in  the  form  of 
innumerable  small  elevations,  which  increase  in  number  and 


height  towards  the  north-east,  where  they  culminate  in  a 
range  of  peaks  whose  altitude  and  rugged  aspect  must  form 
one  of  the  most  terribly  grand  and  romantic  scenes  which 


126  ASTRONOMY. 

imagination  can  conceive.  The  north-east  face  of  the  range 
terminates  abruptly  in  an  almost  vertical  precipice ;  while 
over  the  plain  beneath,  intensely  black  spire-like  shadows 
are  cast,  some  of  which  at  sunrise  extend  full  ninety  miles, 
till  they  lose  themselves  in  the  general  shading  due  to  the 
curvature  of  the  lunar  surface.  Many  of  the  peaks  rise  to 


Fig.  141. 

heights  of  from  eighteen  thousand  to  twenty  thousand  feet 
above  the  plain  at  their  north-east  base  (Fig.  139). 

Fig.  140  represents  an  ideal  lunar  landscape  near  the  base 
of  such  a  lunar  range.  Owing  to  the  absence  of  an  atmos- 
phere, the  stars  will  be  visible  in  full  daylight. 

1 20.  The  Valley  of  the  Alps.  —  The  range  of  the  Alps 
is  shown  in  Fig.  141.  The  great  crater  at  the  north  end  of 
this  range  is  named  Plato.  It  is  seventy  miles  in  diameter. 


ASTRONOMY. 


127 


The  most  remarkable  feature  of  the  Alps  is  the  valley 
near  the  centre  of  the  range.  It  is  more  than  seventy-five 
miles  long,  and  about  six  miles  wide  at  the  broadest  part. 
When  examined  under  favorable  circumstances,  with  a  high 
magnifying  power,  it  is  seen  to  be  a  vast  flat-bottomed 
valley,  bordered  by  gigantic  mountains,  some  of  which 
attain  heights  of  ten  thousand  feet  or  more. 

121.  Isolated  Peaks,  —  There  are  comparatively  few 
isolated  peaks  to  be  found  on  the  surface  of  the  moon. 
One  of  the  most  remarkable  of  these  is  that  known  as  Pico, 


Fig.  142. 

and  shown  in  Fig.  142.  Its  height  exceeds  eight  thousand 
feet,  and  it  is  about  three  times  as  long  at  the  base  as  it  is 
broad.  The  summit  is  cleft  into  three  peaks,  as  is  shown 
by  the  three-peaked  shadow  it  casts  on  the  plain. 

122.  Bright  Rays.  —  About  the  time  of  full  moon,  with 
a  telescope  of  moderate  power,  a  number  of  bright  lines 
may  be  seen  radiating  from  several  of  the  lunar  craters, 
extending  often  to  the  distance  of  hundreds  of  miles. 
These  streaks  do  not  arise  from  any  perceptible  difference 
of  level  of  the  surface,  they  have  no  very  definite  outline, 


128  ASTRONOMY. 

and  they  do  not  present  any  sloping  sides  to  catch  more 
sunlight,  and  thus  shine  brighter,  than  the  general  surface. 
Indeed,  one  great  peculiarity  of  them  is,  that  they  come  out 
most  forcibly  when  the  sun  is  shining  perpendicularly  upon 
them :  hence  they  are  best  seen  when  the  moon  is  at  full 


Fig.  143. 


and  they  are  not  visible  at  all  at  those  regions  upon  which 
the  sun  is  rising  or  setting.  They  are  not  diverted  by  eleva- 
tions in  their  path,  but  traverse  in  their  course  craters, 
mountains,  and  plains  alike,  giving  a  slight  additional  bright- 
ness to  all  objects  over  which  they  pass,  but  producing  no 


ASTRONOMY.  I2Q 

other  effect  upon  them.  "  They  look  as  if,  after  the  whole 
surface  of  the  moon  had  assumed  its  final  configuration,  a 
vast  brush  charged  with  a  whitish  pigment  had  been  drawn 
over  the  globe  in  straight  lines,  radiating  from  a  central 
point,  leaving  its  trail  upon  every  thing  it  touched,  but 
obscuring  nothing." 

The  three  most  conspicuous  craters  from  which  these 
lines  radiate  are  Tycho,  Copernicus*  and  Kepler.  Tycho  is 
seen  at  the  bottom  of  Figs.  143  and  130.  Kepler  is  a  little 
to  the  left  of  Copernicus  in  the  same  figures. 

It  has  been  thought  that  these  bright  streaks  are  chasms 
which  have  been  filled  with  molten  lava,  which,  on  cooling, 
would  afford  a  smooth  reflecting  surface  on  the  top. 

123.  Tycho.  —  This  crater  is  fifty-four  miles  in  diameter, 
and  about  sixteen  thousand  feet  deep,  from  the  highest  ridge 
of  the  rampart  to  the  surface  of  the  plateau,  whence  rises  a 
central  cone  five  thousand  feet  high.  It  is  one  of  the  most 
conspicuous  of  all  the  lunar  craters ;  not  so  much  on 
account  of  its  dimensions  as  from  its  being  the  centre  from 
whence  diverge  those  remarkable  bright  streaks,  many  of 
which  may  be  traced  over  a  thousand  miles  of  the  moon's 
surface  (Fig.  143).  Tycho  appears  to  be  an  instance  of  a 
vast  disruptive  action  which  rent  the  solid  crust  of  the  moon 
into  radiating  fissures,  which  were  subsequently  filled  with 
molten  matter,  whose  superior  luminosity  marks  the  course 
of  the  cracks  in  all  directions  from  the  crater  as  their 
common  centre.  So  numerous  are  these  bright  streaks 
when  examined  by  the  aid  of  the  telescope,  and  they  give 
to  this  region  of  the  moon's  surface  such  increased  lumi- 
nosity, that,  when  viewed  as  a  whole,  the  locality  can  be 
distinctly  seen  at  full  moon  by  the  unassisted  eye,  as  a 
bright  patch  of  light  on  the  southern  portion  of  the  disk. 


130 


ASTRONOMY. 


III.     INFERIOR   AND   SUPERIOR   PLANETS. 
INFERIOR  PLANETS. 

1 24.   The   Inferior  Planets.  —  The    inferior  planets    are 

those  which  lie  be- 
tween the  earth  and 
the  sun,  and  whose 
orbits  are  included 
by  that  of  the  earth. 
They  are  Mercury 
and  Venus. 

125.  Aspects  of  an 
Inferior  Planet.  — 
The  four  chief  aspects 
of  an  inferior  planet 
as  seen  from  the 
earth  are  shown  in 
Fig-  I44'  Fig.  144,  in  which  5 

represents  the  sun,  P  the  planet,  and  E  the  earth. 

When  the  planet  is 
between  the  earth  and 
the  sun,  as  at  P,  it 
is  said  to  be  in  infe- 
rior conjunction. 

When  it  is  in  the 
same  direction  as  the 
sun,  but  beyond  it, 
as  at  P' ',  it  is  said  to 
be  in  superior  con- 
junction. 

When  the  planet  is 
at  such  a  point  in 
its  orbit  that  a  line  Fig- I45- 

drawn  from  the  earth  to  it  would  be  tangent  to  the  orbit, 
as  at  P'  and  P"' }  it  is  said  to  be  at  its  greatest  elongation. 


ASTRONOMY.  131 

126.  Apparent  Motion  of  an  Inferior  Planet.  —  When  the 
planet  is  at  P,  if  it  could  be  seen  at  all,  it  would  appear 
in  the  heavens  at  A.  As  it  moves  from  P  to  P',  it  will 
appear  to  move  in  the  heavens  from  A  to  B.  Then,  as  it 
moves  from  P'  to  P",  it .  will  appear  to  move  back  again 
from  B.to  A.  While  it  moves  from  P"  to  P'",  it  will  appear 
to  move  from  A  to  C;  and,  while  moving  from  P'"  to  P, 


it  will  appear  to  move  back  again  from  C  to  A.  Thus  the 
planet  will  appear  to  oscillate  to  and  fro  across  the  sun  from 
B  to  C,  never  getting  farther  from  the  sun  than  B  on  the 
west,  or  C  on  the  east :  hence,  when  at  these  points,  it  is 
said  to  be  at  its  greatest  western  and  eastern  elongations. 
This  oscillating  motion  of  an  inferior  planet  across  the  sun, 
combined  with  the  sun's  motion  among  the  stars,  causes  the 


132  ASTRONOMY. 

planet  to  describe  a  path  among  the  stars  similar  to  that 
shown  in  Fig.  145. 

127.  Phases  of  an  Inferior  Planet.  —  An  inferior  planet, 
when  viewed  with  a  telescope,  is  found  to  present  a  succes- 
sion of  phases  similar  to  those  of  the  moon.  The  reason 
of  this  is  evident  from  Fig.  146.  As  an  inferior  planet 
passes  around  the  sun,  it  presents  sometimes  more  and 
sometimes  less  of  its  bright  hemisphere  to  the  earth.  When 
the  earth  is  at  T,  and  Venus  at  superior  conjunction,  the 
planet  turns  the  whole  of  its  bright  hemisphere  towards  the 
earth,  and  appears  //////  it  then  becomes  gibbous,  half,  and 
crescent.  When  it  comes  into  inferior  conjunction,  it  turns 

its  dark  hemisphere  towards 
the  earth  :  it  then  becomes 
crescent,  half,  gibbous,  and 
/////  again. 

128.  The  Sidereal  and 
Synodical  Periods  of  an  Infe- 
rior Planet.  —  The  time  it 
takes  a  planet  to  make  a 
complete  revolution  around 
the  sun  is  called  the  side- 
real  period  of  the  planet ; 
and  the  time  it  takes  it  to  pass  from  one  aspect  around  to 
the  same  aspect  again,  its  synodical  period. 

The  synodical  period  of  an  inferior  planet  is  longer  than 
its  sidereal  period.  This  will  be  evident  from  an  examina- 
tion of  Fig.  147.  S  is  the  position  of  the  sun,  E  that  of 
the  earth,  and  P  that  of  the  planet  at  inferior  conjunc- 
tion. Before  the  planet  can  be  in  inferior  conjunction 
again,  it  must  pass  entirely  around  its  orbit,  and  overtake 
the  earth,  which  has  in  the  mean  time  passed  on  in  its  orbit 
to  E'. 

While  the  earth  is  passing  from  E  to  E' ,  the  planet 
passes  entirely  around  its  orbit,  and  from  P  to  P'  in  addition. 


ASTRONOMY.  133 

Now  the  arc  PP'  is  just  equal  to  the  arc  E Er :  hence  the 
planet  has  to  pass  over  the  same  arc  that  the  earth  does, 
and  360°  more.  In  other  words,  the  planet  has  to  gain 
360°  on  the  earth. 

The  synodical  period  of  the  planet  is  found  by  direct 
observation. 

129.  The  Length  of  the  Sidereal  Period.  —  The  length  of 
the  sidereal  period  of  an  inferior  planet  may  be  found  by  the 
following  computation :  — 

Let  a  denote  the  synodical  period  of  the  planet. 
Let  b  denote  the  sidereal  period  of  the  earth, 
Let  x  denote  the  sidereal  period  of  the  planet. 

Then    *-r—  —  the  daily  motion  of  the  earth, 

•^60° 
And      2——  =  the  daily  motion  of  the  planet. 

And      - —       -       ,     =  the  daily  gain  of  the  planet: 

^60° 
Also     « =  the  daily  gain  of  the  planet  : 

360°         360°  _     360° 
Hence —  — j_ —  —   • 


Dividing  by  360°,  we  have  - :  —  -=  =  -; 

Clearing  of  fractions,  we  have  ab  —  ax  —  bx\ 
Transposing  and  collecting,  we  have  (a  +  &)x  =  ab : 

Therefore  x  —  -  ,    ,. 
a  +  b 

1 30.  The  Relative  Distance  of  an  Inferior  Planet.  —  By  the 
relative  distance  of  a  planet,  we  mean  its  distance  from  the  sun 
compared  with  the  earth's  distance  from  the  sun.  The  relative 
distance  of  an  inferior  planet  may  be  found  by  the  following 
method :  — 

Let  V,  in  Fig.  148,  represent  the  position  of  Venus  at  its 
greatest  elongation  from  the  sun,  S  the  position  of  the  sun, 
and  E  that  of  the  earth.  The  line  E  V  will  evidently  be  tan- 
gent to  a  circle  described  about  the  sun  with  a  radius  equal 
to  the  distance  of  Venus  from  the  sun  at  the  time  of  this  great- 


134 


ASTRONOMY. 


est  elongation.  Draw  the  radius  SV  and  the  line  S E.  Since 
SVis  a  radius,  the  angle  at  V  is  a  right  angle.  The  angle 
at  E  is  known  by  measurement,  and  the  angle  at  S  is  equal  to 
90°  —  the  angle  E.  In  the  right-angled  triangle  E  VS,  we  then 
know  the  three  angles,  and  we  wish  to  find  the  ratio  of  the 
side  SVto  the  side  SE. 

The  ratio  of  these  lines  may  be  found  by  trigonometrical 
computation  as  follows  :  — 

VS:  ES  =  s'mSEV:  I. 
Substitute    the  value   of   the   sine    of   SE  V,  and  we  have 

VS:  ES=  .723  :  I. 

Hence  the  relative  distances  of  Venus  and  of  the  earth  from 
the  sun  are  .723  and  i. 

SUPERIOR  PLANETS. 

131.  The  Superior  Planets. — The    superior  planets  are 
those   which    lie   beyond  the   earth.     They  are  Mars,  the 
Asteroids.  Jupiter,  Saturn,  Uranus,  and  Neptune. 

132.  Apparent  Motion  of  a  Superior  Planet.  —  In  order  to 

deduce  the  apparent  motion  of  a  superior 
planet  from  the  real  motions  of  the  earth 
and  planet,  let  S  (Fig.  149)  be  'the  place 
of  the  sun;  I,  2,  3,  etc.,  the  orbit  of  the 
earth ;  a,  b,  c,  etc.,  the  orbit  of  Mars ;  and 
CGL  a  part  of  the  starry  firmament.  Let 
the  orbit  of  the  earth  be  divided  into 
twelve  equal  parts,  each  described  in  one 
month ;  and  let  ab,  be,  cd,  etc.,  be  the 
spaces  described  by  Mars  in  the  same 
time.  Suppose  the  earth  to  be  at  the 
point  i  when  Mars  is  at  the  point  a,  Mars 
will  then  appear  in  the  heavens  in  the 
direction  of  i  a.  When  the  earth  is  at 
3,  and  Mars  at  c,  he  will  appear  in  the 
heavens  at  C.  When  the  earth  arrives 

at  4,  Mars  will  arrive  at  d,  and  will  appear  in  the  heavens  at  D. 

While  the  earth  moves  from  4  to  5  and  from  5  to  6,  Mars  will 


ASTRONOMY. 


135 


appear  to  have  advanced  among  the  stars  from  D  to  E  and 

from    E    to    /", 

in  the  direction 

from     west     to 

east.        During 

the     motion    of 

the  earth  from  6 

to  7  and  from  7 

to  8,  Mars  will 

appear     to     go 

backward    from 

F    to     G     and 

from    G    to   //, 

in  the  direction 

from     east      to 

west.        During 

the     motion    of 

the  earth  from  8 

to  9  and  from  9 

to  10,  Mars  will  Fis-  '49- 

appear  to  advance  from  H  to  /  and  from  /  to  A",  in  the  direction 

from  west  to  east, 
and  the  motion  will 
continue  in  the 
same  direction  until 
near  the  succeeding 
opposition. 

The  apparent  mo- 
tion of  a  superior 
planet  projected  on 
the  heavens  is  thus 
seen  to  be  similar 
to  that  of  an  infe- 
rior planet,  except 
that,  in  the  latter 
case,  the  retrogres- 
Fig-  I5°-  sion  takes  place  near 

inferior   conjunction,    and   in   the   former  it  takes  place  near 

opposition. 


136 


ASTRONOMY. 


133.  Aspects  of  a  Superior  Planet, — The  four  aspects  of 
a  superior  planet  are  shown  in  Fig.  150,  in  which  S  is  the 
position  of  the  sun,  E  that  of  the  earth,  and  P  that  of  the 
planet. 

When  the  plarfet  is  on  the  opposite  side  of  the  earth  to 
the  sun,  as  at  P,  it  is  said  to  be  in  opposition.  The  sun 


Fig.  151. 

and  the  planet  will  then  appear  in  opposite  parts  of  the 
heavens,  the  sun  appearing  at  (7,  and  the  planet  at  A. 

When  the  planet  is  on  the  opposite  side  of  the  sun  to 
the  earth,  as  at  P",  it  is  said  to  be  in  superior  conjunction. 
It  will  then  appear  in  the  same  part  of  the  heavens  as  the 
sun,  both  appearing  at  C. 

When  the  planet  is  at  Pr  and  P" ',  so  that  a  line  drawn 
from  the  earth  through  the  planet  will  make  a  right  angle 
with  a  line  drawn  from  the  earth  to  the  sun,  it  is  said  to  be 


ASTRONOMY. 


137 


in  quadrature.     At  P'  it  is  in  its  western  quadrature,  and  at 
P'"  in  its  eastern  quadrature. 

134.  Phases  of  a  Superior  Planet.  —  Mars   is  the    only 
one  of  the  superior  planets  that   has   appreciable   phases. 
At  quadrature,  as  will  appear  from  Fig.  151,  Mars  does  not 
present  quite  the  same   side  to  the  earth  as  to  the  sun  : 
hence,  near  these  parts  of  its  orbit,  the  planet  appears  slightly 
gibbous.     Elsewhere  in  its  orbit,  the  planet  appears  full. 

All  the  other  superior  planets  are  so  far  away  from  the 
sun  and  earth,  that  the  sides  which  they  turn  towards  the 
sun  and  the  earth  in  every  part  of  their  orbit  are  so  nearly 
the  same,  that  no  change  in  the  form  of  their  disks  can  be 
detected. 

135.  The  Sy nodical  Pe riod 
of  a  Superior  Planet.  —  Dur- 
ing a  synodical  period   of  a 
superior  planet  the  earth  must 
gain  one  revolution,  or  360°, 
on  the  planet,  as  will  be  evi- 
dent from  an  examination  of 
Fig.   152,   in  which  6"  repre- 
sents the   sun,   E   the    earth, 
and  P  the  planet  at  opposi- 
tion.    Before  the   planet  can   be  in   opposition  again,   the 
earth  must  make  a  complete  revolution,  and  overtake  the 
planet,  which  has  in  the  mean  time  passed  on  from  P  to  P'. 

In  the  case  of  most  of  the  superior  planets  the  synodical 
period  is  shorter  than  the  sidereal  period  ;  but  in  the  case 
of  Mars  it  is  longer,  since  Mars  makes  more  than  a  com- 
plete revolution  before  the  earth  overtakes  it. 

The  synodical  period  of  a  superior  planet  is  found  by 
direct  observation. 

136.  The   Sidereal  Period   of  a    Superior    Planet.  —  The 
sidereal  period  of  a  superior  planet  is  found  by  a  method  of 
computation  similar  to  that  for  finding  the  sidereal  period  of 
an  inferior  planet :  — 


Fig.  152. 


138 


ASTRONOMY. 


Let  a  denote  the  synodical  period  of  the  planet, 
Let  b  denote  the  sidereal  period  of  the  earth, 
Let  x  denote  the  sidereal  period  of  the  planet. 

360° 
Then  will  =  daily  motion  of  the  earth, 


And 
Also 
But 
Hence 


360° 

-  =  daily  motion  of  the  planet ; 

360°         360° 

-   =   dailv  gain  of  the  earth. 
b  x 

•360° 

-  =  daily  gain  of  the  earth  : 

360°   _   360°   _    360° 
b  x  a 

I          II 

b        .v        a 
ax  —   ab   —  bx 

(a  —   b]x  —   ab 
ab 


i  -  b' 
137.    The   Relative   Distance   of  a    Superior  Planet. —  Let 


S,  e,  and  ;//,  in  Fig.  153,  represent  the  relative  positions  of  the 
sun,  the  earth,  and  Mars"  when  the  latter  planet  is  in  opposi- 
tion. Let  E  and  M  represent  the  relative  positions  of  the 
earth  and  Mars  the  day  after  opposition.  At  the  first  observa- 
tion Mars  will  be  seen  in  the  direction  em  A,  and  at  the  second 
observation  in  the  direction  EM  A. 

But  the  fixed  stars  are  so  distant,  that  if  a  line,  eA,  were 
drawn  to  a  fixed  star  at  the  first  observation,  and  a  line,  E  B, 
drawn  from  the  earth  to  the  same  fixed  star  at  the  second 


ASTRONOMY.  139 

observation,  these  two  lines  would  be  sensibly  parallel ;  that  is, 
the  fixed  star  would  be  seen  in  the  direction  of  the  line  eA  at 
the  first  observation,  and  in  the  direction  of  the  line  EB,  par- 
allel to  eA,  at  the  second  observation.  But  if  Mars  were  seen 
in  the  direction  of  the  fixed  star  at  the  first  observation,  it  would 
appear  back,  or  west,  of  that  star  at  the  second  observation  by 
the  angular  distance  BE  A;  that  is,  the  planet  would  have 
retrograded  that  angular  distance.  Now,  this  retrogression  of 
Mars  during  one  day,  at  the  time  of  opposition,  can  be  meas- 
ured directly  by  observation.  This  measurement  gives  us  the 
value  of  the  angle  BEAj  but  we  know  the  rate  at  which  both 
the  earth  and  Mars  are  moving  in  their  orbits,  and  from  this 
we  can  easily  find  the  angular  distance  passed  over  by  each  in 
one  day.  This  gives  us  the  angles  ESA  and  MSA.  We  can 
now  find  the  relative  length  of  the  lines  MS  and  ES  (which 
represent  the  distances  of  Mars  and  of  the  earth  from  the  sun), 
both  by  construction  and  by  trigonometrical  computation. 

Since  EB  and  eA  are  parallel,  the  angle  EAS  is  equal 
to  BE  A. 

SEA  =  180°  -  (ESA  +  EAS) 

ESM  =  ESA  -MSA 

EMS  =  180°  -  (SEA  +  ESM). 

We  have  then 

MS  :  ES  =  sin  SEA  :  sin  EMS. 

Substituting  the  values  of  the  sines,  and  reducing  the  ratio 
to  its  lowest  terms,  we  have 

MS  :  ES~  1.524  :  I. 

Thus  we  find  that  the  relative  distances  of  Mars  and  the 
earth  from  the  sun  are  1.524  and  I.  By  the  simple  observation 
of  its  greatest  elongation,  we  are  able  to  determine  the  relative 
distances  of  an  inferior  planet  and  the  earth  from  the  sun  : 
and,  by  the  equally  simple  observation  of  the  daily  retrogres- 
sion of  a  superior  planet,  we  can  find  the  relative  distances  of 
such  a  planet  and  the  earth  from  the  sun. 


ASTRONOMY. 


IV.     THE   SUN. 
I.     MAGNITUDE   AND   DISTANCE   OF   THE    SUN. 

138.  The   Volume  of  the  Sun.  —  The  apparent  diameter 
of  the  sun  is  about  32',  being  a  little  greater  than  that  of 
the  moon.     The  real  diameter  of  the  sun  is  866,400  miles, 
or  about  a  hundred  and  nine  times  that  of  the  earth. 

As  the  diameter  of  the  moon's  orbit  is  only  about  480,000 

miles,  or  some 
sixty  times  the 
diameter  of  the 
earth,  it  follows 
that  the  diameter 
of  the  sun  is 
nearly  double  that 
of  the  moon's 
orbit :  hence,  were 
the  centre  of  the 
sun  placed  at  the 
centre  of  the  earth, 
the  sun  would 
completely  fill  the 
moon's  orbit,  and 
reach  nearly  as 

far  beyond  it  in  every  direction  as  it  is  from  the  earth  to 

the  moon.     The  circumference  of  the  sun  as  compared  with 

the  moon's  orbit  is  shown  in  Fig.  154. 

The  volume  of  the  sun  is   1,305,000  times  that  of  the 

earth. 

139.  The   Mass    of  the  Sun.  —  The    sun    is    much    less 
dense  than  the  earth.     The  mass  of  the  sun  is  only  330,000 
times  that  of  the  earth,  and  its  density  only  about  a  fourth 
that  of  the  earth. 


Fig.  154. 


To  find  the  mass  of  the  sun,  we  first  ascertain  the  distance 


ASTRONOMY.  14! 

the  earth  would  draw  the  moon  towards  itself  in  a  given  time, 
were  the  moon  at  the  distance  of  the  sun,  and  then  form  the 
proportion  :  as  the  distance  the  earth  would  draw  the  moon 
towards  itself  is  to  the  distance  that  the  sun  draws  the  earth 
towards  itself  in  the  same  time,  so  is  the  mass  of  the  earth  to 
the  mass  of  the  sun. 

Although  the  mass  of  the  sun  is  over  three  hundred  thou- 


sand  times  that  of  the  earth,  the  pull  of  gravity  at  the  surface 
of  the  sun  is  only  about  twenty-eight  times  as  great  as  at 
the  surface  of  the  earth.  This  is  because  the  distance  from 
the  surface  of  the  sun  to  its  centre  is  much  greater  than 
from  the  surface  to  the  centre  of  the  earth. 

140.  Size  of  the  Sun  Compared  with  that  of  the  Planets. 
—  The  size  of  the  sun  compared  with  that  of  the  larger 


142 


ASTRONOMY. 


planets  is  shown  in  Fig.  155.  The  mass  of  the  sun  is  more 
than  seven  hundred  and  fifty  times  that  of  all  of  the  planets 
and  moons  in  the  solar  system.  In  Fig.  156  is  shown  the 


Fig.  156. 

apparent  size  of  the  sun  as  seen  from  the  different  planets. 
The  apparent  diameter  of  the  sun  decreases  as  the  distance 
from  it  increases,  and  the  disk  of  the  sun  decreases  as  the 
square  of  the  distance  from  it  increases. 

141.   The   Distance   of  the    Sun. — The    mean    distance 


ASTRONOMY.  143 

of  the  sun  from  the  earth  is  about  92,800,000  miles.  Owing 
to  the  eccentricity  of  the  earth's  orbit,  the  distance  of  the 
sun  varies  somewhat ;  being  about  3,000,000  miles  less  in 
January,  when  the  earth  is  at  perihelion,  than  in  June,  when 
the  earth  is  at  aphelion.  > 

"  But,  though  the  distance  of  the  sun  can  easily  be  stated  in 
figures,  it  is  not  possible  to  give  any  real  idea  of  a  space  so 
enormous  :  it  is  quite  beyond  our  power  of  conception.  If  one 
were  to  try  to  walk  such  a  distance,  supposing  that  he  could 
walk  four  miles  an  hour,  and  keep  it  up  for  ten  hours  every  day, 
it  would  take  sixty-eight  years  and  a  half  to  make  a  single 
million  of  miles,  and  more  than  sixty-three  hundred  years  to 
traverse  the  whole. 

"  If  some  celestial  railway  could  be  imagined,  the  journey  to 
the  sun,  even  if  our  trains  ran  sixty  miles  an  hour  day  and  night 
and  without  a  stop,  would  require  over  a  hundred  and  seventy- 
rive  years.  Sensation,  even,  would  not  travel  so  far  in  a  human 
lifetime.  To  borrow  the  curious  illustration  of  Professor  Men- 
denhall,  if  we  could  imagine  an  infant  with  an  arm  long  enough 
to  enable  hinr  to  touch  the  sun  and  burn  himself,  he  would  die 
of  old  age  Before  the  pain  could  reach  him ;  since,  according 
to  the  experiments  of  Helmholtz  and  others,  a  nervous  shock 
is  communicated  only  at  the  rate  of  about  a  hundred  feet  per 
second,  or  1,637  miles  a  day,  and  would  need  more  than  a  hun- 
dred and  fifty  years  to  make  the  journey.  Sound  would  do  it 
in  about  fourteen  years,  if  it  could  be  transmitted  through  celes- 
tial space ;  and  a  cannon-ball  in  about  nine,  if  it  were  to  move 
uniformly  with  the  same  speed  as  w-hen  it  left  the  muzzle  of 
the  gun.  If  the  earth  could  be  suddenly  stopped  in  her  orbit, 
and  allowed  to  fall  unobstructed  toward  the  sun,  under  the 
accelerating  influence  of  his  attraction,  she  would  reach  the 
centre  in  about  four  months.  I  have  said  if  she  could  be 
stopped ;  but  such  is  the  compass  of  her  orbit,  that,  to  make 
its  circuit  in  a  year,  she  has  to  move  nearly  nineteen  miles  a 
second,  or  more  than  fifty  times  faster  than  the  swiftest  rifle- 
ball  ;  and,  in  moving  twenty  miles,  her  path  deviates  from  per- 
fect straightness  by  less  than  an  eighth  of  an  inch.  And  yet, 
over  all  the  circumference  of  this  tremendous  orbit,  the  sun 


144  ASTRONOMY. 

exercises  his  dominion,  and  every  pulsation  of  his  surface 
receives  its  response  from  the  subject  earth."  l 

142.  Method  of  Finding  the  Sun's  Distance.  —  There  are 
several  methods  of  finding  the  sun's  distance.  The  simplest 
method  is  that  of  finding  the  actual  distance  of  one  of  the 
nearer  planets  by  observing  its  displacement  in  the  sky  as  seen 
from  widely  separated  points  on  the  earth.  As  the  relative 
distances  of  the  planets  from  each  other  and  from  the  sun  are 
well  known,  we  can  easily  deduce  the  actual  distance  of  the 
sun  if  we  can  find  that  of  any  of  the  planets.  The  two  planets 
usually  chosen  for  this  method  are  Mars  and  Venus. 

(i)  The  displacement  of  Mars  in  the  sky,  as  seen  from  two 
observatories  which  differ  considerably  in  latitude,  is,  of  course, 
greatest  when  Mars  is  nearest  the  earth.  Now,  it  is  evident 
than  Mars  will  be  nearer  the  earth  when  in  opposition  than 


Fig-  157- 

when  in  any  other  part  of  its  orbit ;  and  the  planet  will  be  least 
distant  from  the  earth  when  it  is  at  its  perihelion  point,  and  the 
earth  is  at  its  aphelion  point,  at  the  time  of  opposition.  This 
method,  then,  can  be  used  to  the  best  advantage,  when,  at  the 
time  of  opposition,  Mars  is  near  its  perihelion,  and  the  earth 
near  its  aphelion.  These  favorable  oppositions  occur  about 
once  in  fifteen  years,  and  the  last  one  was  in  1877. 

Suppose  two  observers  situated  at  N'  and  S1  (Fig.  157),  near 
the  poles  of  the  earth.  The  one  at  N'  would  see  Mars  in  the 
sky  at  N,  and  the  one  at  S'  would  see  it  at  S.  The  displace- 
ment would  be  the  angle  NMS.  Each  observer  measures 
carefully  the  distance  of  Mars  from  the  same  fixed  star  near 
it.  The  difference  of  these  distances  gives  the  displacement 
of  the  planet,  or  the  angle  NMS.  These  observations  were 
made  with  the  greatest  care  in  1877. 

1  Professor  C.  A.  Young :  The  Sun. 


ASTRONOMY.  145 

(2)  Venus  is  nearest  the  earth  at  the  time  of  inferior  con- 
junction ;  but  it  can  then  be  seen  only  in  the  daytime.  It  is, 
therefore,  impossible  to  ascertain  the  displacement  of  Venus, 
as  seen  from  different  stations,  by  comparing  her  distances 
from  a  fixed  star.  Occasionally,  at  the  time  of  inferior  con- 
junction, Venus  passes  directly  across  the  sun's  disk.  The 
last  of  these  transits  of  Venus  occurred  in  1874,  and  the  next 
will  occur  in  1882.  It  will  then  be  over  a  hundred  years  before 
another  will  occur. 

Suppose  two  observers,  A  and  B  (Fig.  158),  near  the  poles 
of  the  earth  at  the  time  of  a  transit  of  Venus.  The  observer 
at  A  would  see  Venus  crossing  the  sun  at  K,  and  the  one  at 
B  would  see  it  crossing  at  V\.  Any  observation  made  upon 


Fig.  158. 

Venus,  which  would  give  the  distance  and  direction  of  Venus 
from  the  centre  of  the  sun,  as  seen  from  each  station,  would 
enable  us  to  calculate  the  angular  distance  between  the  two 
chords  described  across  the  sun.  This,  of  course,  would  give 
the  displacement  of  Venus  on  the  sun's  disk.  This  method  was 
first  employed  at  the  last  transits  of  Venus  which  occurred 
before  1874;  namely,  those  of  1761  and  1769. 

There  are  three  methods  of  observation  employed  to  ascer- 
tain the  apparent  direction  and  distance  of  Venus  from  the 
centre  of  the  sun,  called  respectively  the  contact  method,  the 
micrometric  method,  and  \.\\Q  photographic  method. 

(a)  In  the  contact  method,  the  observation  consists  in  noting 
the  exact  time  when  Venus  crosses  the  sun's  limb.  To  ascer- 


146  ASTRONOMY. 

tain  this  it  is  necessary  to  observe  the  exact  time  of  external 
and  internal  contact.  This  observation,  though  apparently 
simple,  is  really  very  difficult.  With  reference  to  this  method 
Professor  Young  says,  — 

"  The  difficulties  depend  in  part  upon  the  imperfections  of 
optical  instruments  and  the  human  eye,  partly  upon  the  essen- 
tial nature  of  light  leading  to  what  is  known  as  diffraction,  and 
partly  upon  the  action  of  the  planet's  atmosphere.  The  two 
first-named  causes  produce  what  is  called  irradiation,  and  oper- 
ate to  make  the  apparent  diameter  of  the  planet,  as  seen  on  the 
solar  disk,  smaller  than  it  really  is ;  smaller,  too,  by  an  amount 
which  varies  with  the  size  of  the  telescope,  the  perfection  of 
its  lenses,  and  the  tint  and  brightness  of  the  sun's  image. 

The  edge  of  the  plan- 
et's image  is  also  ren- 
dered slightly  hazy  and 
indistinct. 

"  The  planet's  at- 
mosphere also  causes 
its  disk  to  be  sur- 
rounded by  a  narrow 
ring  of  light,  which 

J59-  becomes    visible   long 

before  the  planet  touches  the  sun,  and,  at  the  moment  of  inter- 
nal contact,  produces  an  appearance,  of  which  the  accompany- 
ing figure  is  intended  to  give  an  idea,  though  on  an  exaggerated 
scale.  The  planet  moves  so  slowly  as  to  occupy  more  than 
twenty  minutes  in  crossing  the  sun's  limb;  so  that  even  if  the 
planet's  edge  were  perfectly  sharp  and  definite,  and  the  sun's 
limb  undistorted,  it  would  be  very  difficult  to  determine  the 
precise  second  at  which  contact  occurs.  But,  as  things  are, 
observers  with  precisely  similar  telescopes,  and  side  by  side, 
often  differ  from  each  other  five  or  six  seconds  ;  and,  where  the 
telescopes  are  not  similar,  the  differences  and  uncertainties  are 
much  greater.  .  .  .  Astronomers,  therefore,  at  present  are 
pretty  much  agreed  that  such  observations  can  be  of  little  value 
in  removing  the  remaining  uncertainty  of  the  parallax,  and  are 
disposed  to  put  more  reliance  upon  the  micrometric  and  photo- 
graphic methods,  which  are  free  from  these  peculiar  difficulties, 


ASTRONOMY.  147 

though,   of   course,  beset  with   others,  which,  however,    it   is 
hoped  will  prove  less  formidable." 

(b)  Of   the   micrometric  method,  as   employed  at  the   last 
transit,  Professor  Young  speaks  as  follows  :  — 

"  The  micrometric  method  requires  the  use  of  a  heliometer, 
—  an  instrument  common  only  in  Germany,  and  requiring  much 
skill  and  practice  in  its  use  in  order  to  obtain  with  it  accurate 
measures.  At  the  late  transit,  a  single  English  party,  two  or 
three  of  the  Russian  parties,  and  all  five  of  the  German,  were 
equipped  with  these  instruments ;  and  at  some  of  the  stations 
extensive  series  of  measures  were  made.  None  of  the  results, 
however,  have  appeared  as  yet ;  so  that  it  is  impossible  to  say 
how  greatly,  if  at  all,  this  method  will  have  the  advantage  in 
precision  over  the  contact  observations." 

(c)  The  following  observations,  with  reference  to  the  photo- 
graphic method,  are  also  taken  from  Professor  Young :  — 

"  The  Americans  and  French  placed  their  main  reliance  upon 
the  photographic  method,  while  the  English  and  Germans  also 
provided  for  its  use  to  a  certain  extent.  The  great  advantage 
of  this  method  is,  that  it  makes  it  possible  to  perform  the 
necessary  measurements  (upon  whose  accuracy  every  thing 
depends)  at  leisure  after  the  transit,  without  hurry,  and  with  all 
possible  precautions.  The  field-work  consists  merely  in  obtain- 
ing as  many  and  as  good  pictures  as  possible.  A  principal 
objection  to  the  method  lies  in  the  difficulty  of  obtaining  good 
pictures,  i.e.,  pictures  free  from  distortion,  and  so  distinct  and 
sharp  as  to  bear  high  magnifying  power  in  the  microscopic 
apparatus  used  for  their  measurement.  The  most  serious  diffi- 
culty, however,  is  involved  in  the  accurate  tletermination  of  the 
scale  of  the  picture  ;  that  is,  of  the  number  of  seconds  of  arc 
corresponding  to  a  linear  inch  upon  the  plate.  Besides  this, 
we  must  know  the  exact  Greenwich  time  at  which  each  picture 
is  taken,  and  it  is  also  extremely  desirable  that  the  orientation 
of  the  picture  should  be  accurately  determined;  that  is,  the 
north  and  south,  the  east  and  west  points  of  the  solar  image  on 
the  finished  plate.  There  has  been  a  good  deal  of  anxiety  lest 
the  image,  however  accurate  and  sharp  when  first  produced, 
should  alter,  in  course  of  time,  through  the  contraction  of  the 
collodion  film  on  the  glass  plate:  but  the  experiments  of 


148 


ASTRONOMY. 


Rutherfurd,  Huggins,  and  Paschen,  seem  to  show  that  this 
danger  is  imaginary.  .  .  .  The  Americans  placed  the  photo- 
graphic telescope  exactly  in  line  with  a  meridian  instrument, 
and  so  determined,  with  the  extremest  precision,  the  direction 
in  which  it  was  pointed.  Knowing  this  and  the  time  at  which 
any  picture  was  taken,  it  becomes  possible,  with  the  help  of 
the  plumb-line  image,  to  determine  precisely  the  orientation  of 
the  picture,  —  an  advantage  possessed  by  the  American  pictures 
alone,  and  making  their  value  nearly  twice  as  great  as  otherwise 
it  would  have  been. 

"  The  rigure  below  is  a  representation  of  one  of  the  Ameri- 
can photographs  re- 
duced about  one- 
half.  V  is  the 
image  of  Venus, 
which,  on  the  actual 
plate,  is  about  a 
seventh  of  an  inch 
in  diameter;  a  a'  is 
the  image  of  the 
plumb-line.  The 
centre  of  the  reticle 
is  marked  with  a 
cross/' 

The  English  pho- 
tographs proved  to 
be  of  little  value, 
and  the  results  of 

the  measurements  and  calculations  upon  the  American  pictures 
have  not  yet  been  published.  There  is  a  growing  apprehension 
that  no  photographic  method  can  be  relied  upon. 

The  most  recent  determinations  by  various  methods  indi- 
cate that  the  sun's  distance  is  such  that  his  parallax  is  about 
eighty-eight  seconds.  This  would  make  the  linear  value  of 
a  second  at  the  surface  of  the  sun  about  four  hundred  and 
fifty  miles. 


PLATE  I. 


ASTRONOMY.  149 

II.     PHYSICAL    AND    CHEMICAL    CONDITION    OF 
THE    SUN. 

PHYSICAL  CONDITION  OF  THE  SUN. 

143.  The  Sun   Composed  mainly  of  Gases.  —  It  is  now 
generally  believed  that  the  sun  is  mainly  a  ball  of  gas,  or 
vapor,  powerfully  condensed  at  the  centre  by  the  weight 
of  the  superincumbent  mass,  but  kept  from  liquefying  by  its 
exceedingly  high  temperature. 

The  gaseous  interior  of  the  sun  is  surrounded  by  a  layer 
of  luminous  clouds,  which  constitutes  its  visible  surface,  and 
which  is  called  its  photosphere.  Here  and  there  in  the 
photosphere  are  seen  dark  spots,  which  often  attain  an 
immense  magnitude. 

These  clouds  float  in  the  solar  atmosphere,  which  extends 
some  distance  beyond  them. 

The  luminous  surface  of  the  sun  is  surrounded  by  a  rose- 
colored  stratum  of  gaseous  matter,  called  the  chromosphere. 
Here  and  there  great  masses  of  this  chromospheric  matter 
rise  high  above  the  general  level.  These  masses  are  called 
prominences. 

Outside  of  the  chromosphere  is  the  corona,  an  irregular 
halo  of  faint,  pearly  light,  mainly  composed  of  filaments 
and  streamers,  which  radiate  from  the  sun  to  enormous  dis- 
tances, often  more  than  a  million  of  miles. 

In  Fig.  161  is  shown  a  section  of  the  sun,  according  to 
Professor  Young. 

The  accompanying  lithographic  plate  gives  a  general  view 
of  the  photosphere  with  its  spots,  and  of  the  chromosphere 
and  its  prominences. 

144.  The  Temperature   of  the  Sun.  —  Those  who    have 
investigated  the  subject  of  the  temperature  of  the  sun  have 
come  to  very  different  conclusions ;  some  placing  it  as  high 
as   four  million   degrees   Fahrenheit,   and  others  as  low  as 
ten  thousand  degrees.     Professor  Young  thinks  that  Rosetti's 


ISO 


ASTRONOMY. 


estimate  of  eighteen  thousand  degrees  as  the  effective  tem- 
perature of  the  sun's  surface  is  probably  not  far  from 
correct.  By  this  is  meant  the  temperature  that  a  uniform 
surface  of  lampblack  of  the  size  of  the  sun  must  have  in 
order  to  radiate  as  much  heat  as  the  sun  does.  The  most 
intense  artificial  heat  does  not  exceed  four  thousand  degrees 
Fahrenheit. 


Fig.  161. 

145.  The  Amount  of  Heat  Radiated  by  the  Sun. — A 
unit  of  heat  is  the  amount  of  heat  required  to  raise  a 
pound  of  water  one  degree  in  temperature.  It  takes  about 
a  hundred  and  forty-three  units  of  heat  to  melt  a  pound  of 
ice  without  changing  its  temperature.  A  cubic  foot  of  ice 
weighs  about  fifty-seven  pounds.  According  to  Sir  William 
Herschel,  were  all  the  heat  radiated  by  the  sun  concentrated 


ASTRONOMY. 


on  a  cylinder  of  ice  forty-five  miles  in  diameter,  it  would 
melt  it  off  at  the  rate  of  about  a  hundred  and  ninety  thou- 
sand miles  a  second. 

Professor  Young  gives  the  following  illustration  of  the 
energy  of  solar  radiation :  "  If  we  could  build  up  a  solid 
column  of  ice  from  the  earth  to  the  sun,  two  miles  and  a 
quarter  in  diameter,  spanning  the  inconceivable  abyss  of 
ninety-three  million  miles,  and  if  then  the  sun  should  con- 
centrate his  power  upon  it,  it  would  dissolve  and  melt,  not 
in  an  hour,  nor  a  minute,  but  in  a  single  second.  One 
swing  of  the  pendulum,  and  it  would  be  water ;  seven  more, 
and  it  would  be  dissipated  in  vapor." 

This  heat  would  be 
sufficient  to  melt  a  layer 
of  ice  nearly  fifty  feet 
thick  all  around  the  sun 
in  a  minute.  To  develop 
this  heat  would  require 
the  hourly  consumption 
of  a  layer  of  anthracite 
coal,  more  than  sixteen 
feet  thick,  over  the  entire 
surface  of  the  sun ;  and  the  mechanical  equivalent  of  this 
heat  is  about  ten  thousand  horse-power  on  every  square 
foot  of  the  sun's  surface. 

146.  The  Brightness  of  the  Surfs  Surface.  —  The  sun's 
surface  is  a  hundred  and  ninety  thousand  times  as  bright  as 
a  candle-flame,  a  hundred  and  forty-six  times  as  bright  as 
the  calcium-light,  and  about  three  times  and  a  half  as  bright 
as  the  voltaic  arc. 

The  sun's  disk  is  much  less  bright  near  the  margin  than 
near  the  centre,  a  point  on  the  limb  of  the  sun  being  only 
about  a  fourth  as  bright  as  one  near  the  centre  of  the  disk. 
This  diminution  of  brightness  towards  the  margin  of  the 
disk  is  due  to  the  increase  in  the  absorption  of  the  solar 


Fig.  162. 


152  ASTRONOMY. 

atmosphere  as  we  pass  from  the  centre  towards  the  margin 
of  the  sun's  disk ;  and  this  increased  absorption  is  due  to 
the  fact,  that  the  rays  which  reach  us  from  near  the  margin 
have  to  traverse  a  much  greater  thickness  of  the  solar 
atmosphere  than  those  which  reach  us  from  the  centre  of 
the  disk.  This  will  be  evident  from  Fig.  162,  in  which  the 
arrows  mark  the  paths  of  rays  from  different  parts  of  the 
solar  disk. 

THE  SPECTROSCOPE. 

147.  The  Spectroscope  as  an  Astronomical  Instrument. 
—  The  spectroscope  is  now  continually  employed  in  the 
study  of  the  physical  condition  and  chemical  constitution 


Fig.  163. 

of  the  sun  and  of  the  other  heavenly  bodies.  It  has 
become  almost  as  indispensable  to  the  astronomer  as  the 
telescope. 

148.  The  Dispersion  Spectroscope. — The  essential  parts 
of  the  dispersion  spectroscope  are  shown  in  Fig.  163. 
These  are  the  collimator  tube,  the  prism,  and  the  telescope. 
The  collimator  tube  has  a  narrow  slit  at  one  end,  through 
which  the  light  to  be  examined  is  admitted,  and  some- 
where within  the  tube  a  lens  for  condensing  the  light.  The 
light  is  dispersed  on  passing  through  the  prism  :  it  then 
passes  through  the  objective  of  the  telescope,  and  forms 


ASTRONOMY. 


153 


within  the  tube  an  image  of  the  spectrum,  which  is  exam- 
ined by  means  of  the  eye-piece.  The  power  of  the  spec- 
troscope is  increased  by  increasing  the  number  of  prisms, 
which  are  arranged  so 
that  the  light  shall  pass 
through  one  after  an- 
other in  succession. 
Such  an  arrangement 
of  prisms  is  shown  in 
Fig.  164.  One  end 
of  the  collimator  tube 
is  seen  at  the  left,  and 
one  end  of  the  tele- 
scope at  the  right. 
Sometimes  the  prisms 
are  made  long,  and 
the  light  is  sent  twice 
through  the  same  train 
of  prisms,  once  through 
the  lower,  and  once 

through  the    upper,  half  of  the   prisms.     This    is   accom- 
plished   by   placing   a   rectangular   prism    against    the    last 

prism  of  the  train, 
as  shown  in  Fig. 
165. 

149.  The  Mi- 
crometer Scale.  — 
Various  devices  are 
employed  to  obtain 
an  image  of  a  mi- 
crometer scale  in 
the  tube  of  the 
telescope  beside  that  of  the  spectrum. 

One    of    the    simplest    of    these    methods   is    shown    in 
Fig.  1 66.     A  is  the  telescope,  B  the  collimator,  and  C  the 


Fig.  165. 


154 


ASTRONOMY. 


micrometer  tube.  The  opening  at  the  outer  end  of  C  con- 
tains a  piece  of  glass  which  has  a  micrometer  scale  marked 
upon  it.  The  light  from  the  candle  shines  through  this 


Fig.  166. 

glass,  falls  upon  the  surface  of  the  prism  P,  and  is  thence 
reflected    into    the   telescope,    where   it  forms   an   enlarged 


Fig.  167. 

image  of  the  micrometer  scale  alongside  the  image  of  the 
spectrum. 

150.   The   Comparison   of  Spectra.  —  In    order  to  com- 


ASTRONOMY. 


155 


pare   two  spectra,  it  is  desirable  to  be  able  to   see   them 

side  by  side  in  the  telescope.     The  images  of  two  spectra 

may  be  obtained  side  by  side  in  the  telescope  tube  by  the 

use   of  a   little   rectangular   prism,  which   covers  one-half 

of  the   slit  of  the  collimator  tube,  as  shown  in  Fig.   167. 

The   light   from   one    source    is   admitted    directly  through 

the    uncovered    half  of  the   slit,  while    the  light  from  the 

other  source  is  sent 

through  the  covered 

portion    of   the    slit 

by     reflection    from 

the    surface    of    the 

rectangular       prism.  Fis-  l68- 

This  arrangement  and  its  action  will  be  readily  understood 

from  Fig.  167. 

151.  Direct-Vision  Spectroscope.  —  Abeam  of  light  may 
be  dispersed,  without  any  ultimate  deflection  from  its 
course,  by  combining  prisms  of  crown  and  flint  glass  with 
equal  refractive,  but  unequal  dispersive  powers.  Such  a 
combination  of  prisms  is  called  a  direct-vision  combination. 


One  of  three  prisms  is  shown  in  Fig.  168,  and  one  of  five 
prisms  in  Fig.  169. 

A  direct-vision  spectroscope  (Fig.  170)  is  one  in  which 
a  direct-vision  combination  of  prisms  is  employed.  C  is 
the  collimator  tube,  P  the  train  of  prisms,  F  the  telescope, 
and  r  the  comparison  prism. 

152.  The  Telespectroscope. — The  spectroscope,  when 
used  for  astronomical  work,  is  usually  combined  with  a 


56 


ASTRONOMY. 


telescope.  The  compound  instrument  is  called  a  telespec- 
troscope.  The  spectroscope  is  mounted  at  the  end  of  the 
telescope  in  such  a  way  that  the  image  formed  by  the 


Fig.  170, 

object-glass  of  the  telescope  falls  upon  the  slit  at  the  end 
of  the  collimator  tube.  A  telespectroscope  of  small  dis- 
persive power  is  shown  in  Fig.  171  ;  a  being  the  object- 
glass  of  the  telescope,  cc  the  tube  of  the  telescope,  and 


Fig.  171. 

e  the  comparison  prism  at  the  end  of  the  collimator  tube. 
A  more  powerful  instrument  is  shown  in  Fig.  172.  A  is 
the  telescope,  C  the  collimator  tube  of  the  spectroscope, 


ASTRONOMY. 


157 


P  the  train  of  prisms,  and  E  the  telescope  tube.  Fig.  1 73 
shows  a  still  more  powerful  spectroscope  attached  to  the 
great  Newall  refractor  (18). 


Fig.  172. 


153.   The  Diffraction  Spectroscope.  —  A  diffraction  spec- 
troscope  is    one    in  which    the    spectrum    is   produced    by 


Fig.  173- 

reflection  of  the  light  from  a  finely  ruled  surface,  or  grating, 
as  it  is  called,  instead  of  by  dispersion  in  passing  through  a 


158  ASTRONOMY. 

prism.  The  essential  parts  of  this  instrument  are  shown 
in  Fig  174.  This  spectroscope  may  be  attached  to  the 
telescope  in  the  same  manner  as  the  dispersion  spectroscope. 
When  the  spectroscope  is  thus  used,  the  eye-piece  of  the 
telescope  is  removed. 

SPECTRA. 

154.  Continuous  Spectra.  —  Light  from  an  incandescent 
solid  or  liquid  which  has  suffered  no  absorption  in  the 
medium  which  it  has  traversed  gives  a  spectrum  consisting 
of  a  continuous  colored  band,  in  which  the  colors,  from 
the  red  to  the  violet,  pass  gradually  and  imperceptibly  into 


Fig.  174. 

one  another.     The  spectrum  is  entirely  free  from  either  light 
or  dark  lines,  and  is  called  a  continuous  spectrum. 

155.  Bright-Lined  Spectra.  —  Light  from  a  luminous  gas 
or  vapor  gives  a  spectrum  composed  of  bright  lines  sepa- 
rated by  dark  spaces,  and  known  as  a  bright-lined  spec- 
trum. It  has  been  found  that  the  lines  in  the  spectrum  of 
a  substance  in  the  state  of  a  gas  or  vapor  are  the  most 
characteristic  thing  about  the  substance,  since  no  two  vapors 
give  exactly  the  same  lines  :  hence,  when  we  have  once 
become  acquainted  with  the  bright-lined  spectrum  of  any 
substance,  we  can  ever  after  recognize  that  substance  by 
the  spectrum  of  its  luminous  vapor.  Even  when  several 
substances  are  mixed,  they  may  all  be  recognized  by  the 
bright-lined  spectrum  of  the  mixture,  since  the  lines  of 


ASTRONOMY. 


159 


all  the  substances  will  be  present  in  the  spectrum  of  the 
mixture.  This  method  of  identifying  substances  by  their 
spectra  is  called  spectrum  analysis. 

The  bright-lined  spectra  of  several  substances  are  given 
in  the  frontispiece.  The  number  of  lines  in  the  spectra  of 
the  elements  varies  greatly.  The  spectrum  of  sodium  is  one 
of  the  simplest,  while  that  of  iron  is  one  of  the  most  com- 
plex. The  latter  contains  over  six  hundred  lines.  Though 
no  two  vapors  give  identical  spectra,  there  are  many  cases 
in  which  one  or  more  of  the  spectral  lines  of  one  element 
coincide  in  position  with  lines  of  other  elements. 

156.  Methods  of  rendering  Gases  and  Vapors  Luminous. — 
In  order  to  study  the  spectra  of  vapors  and  gases  it  is  neces- 
sary to  have  some  means  of  converting  solids  and  liquids  into 
vapor,  and  also  of  rendering 
the  vapors  and  gases  lumi- 
nous. There  are  four  meth- 
ods of  obtaining  luminous 
vapors  and  gases  in  common 
use. 

(1)  By  means  of  tJie  Bun- 
's en  Flame. —  This  is  a  very 
hot     but     an     almost     non- 
luminous     flame.        If     any 
readily  volatilized  substance, 
such    as    the    compounds    of 
sodium,    calcium,    strontium, 
etc.,  is   introduced   into   this 
flame  on  a  fine  platinum  wire, 
it  is  volatilized  in  the  flame, 
and    its    vapor    is    rendered 

luminous,  giving  the  flame  its  own  peculiar  color.  The  flame 
thus  colored  may  be  examined  by  the  spectroscope.  The 
arrangement  of  the  flame  is  shown  in  Fig.  175. 

(2)  By  means   of  the   Voltaic   Arc.  —  Kn   electric   lamp  is 
shown  in  Fig.  176.     When  this  lamp  is  to  be  used  for  obtain- 
ing luminous  vapors,  the  lower  carbon  is  made  larger  than  the 


Fig.  175- 


i6o 


ASTRONOMY. 


upper  one,  and  hollowed  out  at  the  top  into  a  little  cup.  The 
substance  to  be  volatilized  is  placed  in  this  cup,  and  the  current 
is  allowed  to  pass.  The  heat  of  the  voltaic  arc  is  much  more 
intense  than  that  of  the  Bunsen  flame :  hence  substances  that 

cannot  be  volatil- 
ized in  the  flame 
are  readily  vola- 
tilized in  the  arc, 
and  the  vapor 
formed  is  raised 
to  a  very  high 
temperature. 

(3)  By  means 
of  the  Spark 
from  an  Induc- 
.  tion  CoiL—  The 
arrangement  of 
the  coil  for  ob- 
taining luminous 
vapors  is  shown 
in  Fig.  177. 

The  terminals 
of  the  coil  be- 
tween which  the 
.  spark  is  to  pass 
are  brought  quite 
close  together. 
When  we  wish 
to  vaporize  any 
^  metal,  as  iron, 
the  terminals  are 
made  of  iron. 
On  the  passage 
of  the  spark,  a 
little  of  the  iron  at  the  ends  of  the  terminals  is  evaporated; 
and  the  vapor  is  rendered  luminous  in  the  space  traversed  by 
the  spark.  A  condenser  is  usually  placed  in  the  circuit.  With 
the  coil,  the  temperature  may  be  varied  at  pleasure ;  and  the 
vapor  may  be  raised  even  to  a  higher  temperature  than  with 


Fig.  176. 


ASTRONOMY. 


161 


the  electric  lamp.  To  obtain  a  low  temperature,  the  coil  is 
used  without  the  condenser.  By  using  a  larger  and  larger 
condenser,  the  temperature  may  be  raised  higher  and  higher. 

By  means  of  the  induction  coil  we  may  also  heat  gases  to 
incandescence.  It  is  only  necessary  to  allow  the  spark  to 
pass  through  a  space  filled  with,  the  gas. 

(4)  By  means  of  a  Vacuum  Tube.  —  The  form  of  the  vacuum 
tube  commonly  used  for  this  purpose  is  shown  in  Fig.  178. 
The  gas  to  be  examined,  and  which  is  contained  in  the  tube, 
has  very  slight  density ;  but  upon  the  passage  of  the  discharge 
from  an  induction 
coil  or  a  Holtz 

machine,    through  r. 

the  tube,  the  gas 
in  the  capillary 
part  of  the  tube 
becomes  heated  to 
a  high  tempera- 
ture, and  is  then 
quite  brilliant. 


157.  Reversed 
Spectra.  — \i  the 
light  from  an  in- 
candescent cylin- 
der of"  lime,  or 
from  the 


Fig.  177- 

incan- 
descent point  of  an  electric  lamp,  is  allowed  to  pass 
through  luminous  sodium  vapor,  and  is  then  examined  with 
a  spectroscope,  the  spectrum  will  be  found  to  be  a  bright 
spectrum  crossed  by  a  single  'dark  line  in  the  position  of 
the  yellow  line  of  the  sodium  vapor.  The  spectrum  of 
sodium  vapor  is  reversed,  its  bright  lines  becoming  dark 
and  its  dark  spaces  bright.  With  a  spectroscope  of  any 
considerable  power,  the  yellow  line  of  sodium  vapor  is 
resolved  into  a  double  line.  With  a  spectroscope  of  the 
same  power,  the  dark  sodium  line  of  the  reversed  spectrum 
is  seen  to  be  a  double  line. 


1 62  ASTRONOMY. 

It  is  found  to  be  generally  true,  that  the  spectrum  of  the 
light  from  an  incandescent  solid  or  liquid  which  has  passed 
through  a  luminous  vapor  on  its  way  to  the  spectroscope 
is  made  up  of  a  bright  ground  crossed  by  dark  lines ;  there 

being  a  dark  line  for  every  bright  line  that  the 

vapor  alone  would  give. 

158.  Explanation   of  Reversed  Spectra.  —  It 
has  been  found  that  gases  absorb  and  quench  rays 
of    the  same    degree    of    refrangibility    as    those 
which    they    themselves    emit,    and    no    others. 
When    a    solid   is   shining   through   a  luminous 
vapor,  this  absorbs  and  quenches  those  rays  from 
the  solid  which  have  the  same  degrees  of  refrangi- 
bility as  those  which  it  is  itself  emitting:  hence 
the  lines  of  the  spectrum  receive  light  from  the 
vapor  alone,  while  the  spaces  between  the  lines 
receive   light  from   the  solid.     Now,  solids   and 
liquids,  when  heated  to  incandescence,  give  a  very 
much  brighter  light  than  vapors  and  gases  at  the 
same  temperature :  hence  the  lines  of  a  reversed 
spectrum,  though  receiving  light,  from  the  vapor 
or  gas,  appear  dark  by  contrast. 

159.  Effect   of  Increasing   the    Power  of  the 
Spectroscope  npon  tJie  Brilliancy  of  a  Spectrum. 
—  An  increase  in  the   power  of  a   spectroscope 
diminishes    the    brilliancy  of   a   continuous   spec- 
trum, since  it  makes  the  colored  band  longer,  and 
therefore   spreads   the   light   out   over  a  greater 
extent  of  surface;   but,  in  the  case  of  a  bright- 
lined  spectrum,  an  increase  of  power  in  the  spec- 
troscope produces  scarcely  any  alteration  in  the 

brilliancy  of  the  lines,  since  it  merely  separates  the  lines  far- 
ther without  making  the  lines  themselves  any  wider.  In  the 
case  of  a  reversed  spectrum,  an  increase  of  power  in  the  spec- 
troscope dilutes  the  light  in  the  spaces  between  the  lines 
without  diluting  that  of  the  lines :  hence  lines  which  appear 
dark  in  a  spectroscope  of  slight  dispersive  power  may  appear 
bright  in  an  instrument  of  great  dispersive  power. 


ASTRONOMY. 


160.  Change  of  the  Spectrum  with  the  Density  of  the  Lu- 
minous Vapor.  —  It  has  been  found,  that,  as  the  density  of  a 
luminous  vapor  is  diminished,  the  lines  in   its   spectrum  be- 
come   fewer    and    fewer,   till 

they  are  finally  reduced  to 
one.  On  the  other  hand,  an 
increase  of  density  causes 
new  lines  to  appear  in  the 
spectrum,  and  the  old  lines  to  Omnrjt, 
become  thicker. 

161.  Change    of  the    Spec- 
trum  with   the    Temperature 
of  the    Luminous     Vapor. — 
It   has   also  been  found  that 
the   appearance   of   a   bright- 
lined   spectrum  changes  con- 
siderably  with    the    tempera- 
ture of   the   luminous   vapor. 
In   some   cases,   an   increase 
of    temperature    changes   the 
relative     intensities     of     the 
lines  ;  in  other  cases,  it  causes 
new  lines  to  appear,  and  old 
lines  to  disappear. 

In  the  case  of  a  com- 
pound vapor,  an  increase 
of  temperature  causes  the 
colored  bands  (which  are 
peculiar  to  the  spectrum  of 
the  compound)  to  disappear, 
and  to  be  replaced  by  the 
spectral  lines  of  the  ele- 
ments of  which  the  com- 
pound is  made  up.  The 
heat  appears  to  dissociate  Flg-  I79> 

the  compound;  that  is,  to  resolve  it  into  its  constituent 
elements.  In  this  case,  each  elementary  vapor  would 'give  its 
own  spectral  lines.  As  the  compound  is  not  completely  dis- 
sociated at  once,  it  is  possible,  of  course,  for  one  or  more  of 


164 


ASTRONOMY. 


the  spectral  lines  of  the  elementary  vapors  to  co-exist  in  the 
spectrum  with  the  bands  of  the  compound. 

It  has  been  found,  that,  in  some  cases,  the  spectra  of  the 
elementary  gases  change  with  the  temperature  of  the  gas;  and 
Lockyer  thinks  he  has  discovered  conclusive  evidence,  in  the 
spectra  of  the  sun  and  stars,  that  many  of  the  substances 
regarded  as  elementary  are  really  resolved  into  simpler  sub- 
stances by  the  intense  heat  of  the  sun ;  in  other  words,  that  our 
so-called  elements  are  really  compounds. 

CHEMICAL  CONSTITUTION  OF  THE  SUN. 

162.   The    Solar    Spectrum.  —  The    solar    spectrum    is 
crossed  transversely  by 
and    hence    it    belongs 


a  great  number  of  fine  dark  lines, 
to    the    class    of    reversed  spectra. 


Fig.  180. 

These  lines  were  first  studied  and  mapped  by  Fraun- 
hofer,  and  from  him  they  have  been  called  Fraunhofer's 
lines. 

A  reduced  copy  of  Fraunhofers  map  is  shown  in  Fig.  179. 
A  few  of  the  most  prominent  of  the  dark  solar  lines  are 
designated  by  the  letters  of  the  alphabet.  The  other  lines  are 
usually  designated  by  the  numbers  at  which  they  are  found  on 
the  scale  which  accompanies  the  map.  This  scale  is  usually 
drawn  at  the  top  of  the  map,  as  will  be  seen  in  some  of  the 
following  diagrams.  The  two  most  elaborate  maps  of  the  solar 
spectrum  are  those  of  Kirchhoff  and  Angstrom.  The  scale  on 
KirchhofFs  map  is  an  arbitrary  one,  while  that  of  Angstrom  is 
based  upon  the  wave-lengths  of  the  rays  of  light  which  would 
fall  upon  the  lines  in  the  spectrum. 

The   appearance    of   the   spectrum  varies  greatly  with  the 


ASTRONOMY. 


i6S 


power  of  the  spectroscope  employed.  Fig.  180  shows  a  por- 
tion of  the  spectrum  as  it  appears  in  a  spectroscope  of  a  single 
prism:  while  Fig.  181  shows  the  b  group  of  lines  alone,  as  they 
appear  in  a  powerful  diffraction  spectroscope. 

163.  The  Telluric  Lines. — There  are  many  lines  of  the 
solar  spectrum  which  vary  considerably  in  intensity  as  the 
sun  passes  from  the  horizon  to  the  meridian,  being  most 
intense  when  the  sun  is  nearest  the  horizon,  and  when  his 
rays  are  obliged  to  pass  through  the  greatest  depth  of  the 
earth's  atmosphere.  These  lines  are  of  atmospheric  origin, 
and  are  due  to  the  absorption  of  the  aqueous  vapor  in  our 
atmosphere.  They  are  the  same  lines  that  are  obtained 


Fig.  181. 

when  a  candle  or  other  artificial  light  is  examined  with  a 
spectroscope  through  a  long  tube  filled  with  steam.  Since 
these  lines  are  due  to  the  absorption  of  our  own  atmos- 
phere, they  are  called  telluric  lines.  A  map  of  these  lines 
is  shown  in  Fig.  182. 

164.  The  Solar  Lines.  —  After  deducting  the  telluric 
lines,  the  remaining  lines  of  the  solar  spectrum  are  of  solar 
origin.  They  must  be  due  to  absorption  which  takes  place 
in  the  sun's  atmosphere.  They  are,  in  fact,  the  reversed 
spectra  of  the  elements  which  exist  in  the  solar  atmosphere 
in  the  state  of  vapor :  hence  we  conclude  that  the  luminous 
surface  of  the  sun  is  surrounded  with  an  atmosphere  of 
luminous  vapors.  The  temperature  of  this  atmosphere,  at 


i66 


ASTRONOMY. 


least  near  the  surface  of  the  sun,  must  be  sufficient  to 
enable  all  the  elements  known  on  the 
earth  to  exist  in  it  as  vapors. 

165.  Chemical  Constitution  of  the 
Sun's  Atmosphere.  —  To  find  whether 
any  element  which  exists  on  the  earth 
is  present  in  the  solar  atmosphere,  we 
have  merely  to  ascertain  whether  the 
bright  lines  of  its  gaseous  spectrum 
are  matched  by  dark  lines  in  the 
solar  spectrum  when  the  two  spectra 
are  placed  side  by  side.  In  Fig.  183, 
we  have  in  No.  i  a  portion  of  the 
red  end  of  the  solar  spectra,  and  in 
No.  2  the  spectrum  of  sodium  vapor, 
both  as  obtained  in  the  same  spec- 
troscope by  means  of  the  compari- 
son prism.  It  will  be  seen  that  the 
double  sodium  line  is  exactly  matched 
by  a  double  dark  line  of  the  solar 
spectrum :  hence  we  conclude  that 
sodium  vapor  is  present  in  the  sun's 
atmosphere.  Fig.  184  shows  the 
matching  of  a  great  number  of  the 
bright  lines  of  iron  vapor  by  dark 
lines  in  the  solar  spectrum.'  This 
matching  of  the  iron  lines  establishes 
the  fact  that  iron  vapor  is  present  in 
the  solar  atmosphere. 

The    following    table  (given   by  Pro- 
fessor Young)  contains  a  list  of  all  the 
elements  which  have,  up  to  the  present 
Fig.  182.  time,    been    detected   with    certainty    in 

the  sun's  atmosphere.     It  also  gives  the  number  of  bright  lines 
in  the  spectrum   of   each    element,  and  the  number  of   those 


ASTRONOMY. 


i67 


lines  which    have   been  matched   by  dark   lines   in   the   solar 
spectrum :  — 


ELEMENTS. 

Bright  Lines 
in  Spectrum. 

Lines  Reversed 
in  Solar 
Spectrum. 

Observer. 

600 

460 

Kirchhoff. 

2.  Titanium  .     .     . 

206 

118 

Thalen. 

3.  Calcium     .     .     . 

89 

75 

Kirchhoff. 

4.  Manganese     .     . 

75 

57 

Angstrom. 

5.  Nickel  .    . 

5i 

33 

Kirchhoff. 

6.  Cobalt  .... 

86 

19 

Thalen. 

7.  Chromium      .     . 

7i 

18 

Kirchhoff. 

8.  Barium  .... 

26 

ii 

Kirchhoff. 

9.  Sodium      .     .     . 

9 

9 

Kirchhoff. 

10.  Magnesium    .     . 

7 

7 

Kirchhoff. 

ii.  Copper?    .     .     , 

IS 

7? 

Kirchhoff. 

12.  Hydrogen.     .     . 

5 

5 

Angstrom. 

13.  Palladium.     .     . 

29 

5 

Lockyer. 

14.  Vanadium  .     .     . 

54 

4 

Lockyer. 

15.  Molybdenum.     . 

27 

4 

Lockyer. 

1  6.  Strontium  . 

74 

4 

Lockyer. 

17.  Lead     .... 

4i 

3 

Lockyer. 

18.  Uranium   .     .     . 

21 

3 

Lockyer. 

19.  Aluminium     . 

M 

2 

Angstrom. 

20.  Cerium.     .     .     . 

64 

2 

Lockyer. 

21.  Cadmium  .     .     . 

20 

2 

Lockyer. 

Oxygen  a  >     .     . 

42 

12  ±  bright 

H.  Draper. 

22.     ,-.                   ^  r 

Oxygen  p  >     .     . 

4 

4? 

Schuster. 

In  addition  to  the  above  elements,  it  is  probable  that  several 
other  elements  are  present  in  the  sun's  atmosphere ;  since  at 
least  one  of  their  bright  lines  has  been  found  to  coincide  with 
dark  lines  of  the  solar  spectrum.  There  are,  however,  a  large 
number  of  elements,  no  traces  of  which  have  yet  been  detected  ; 
and,  in  the  cases  of  the  elements  whose  presence  in  the  solar 
atmosphere  has  been  established,  the  matching  of  the  lines  is 
far  from  complete  in  the  majority  of  the  cases,  as  will  be  seen 
from  the  above  table.  This  want  of  complete  coincidence  of 
the  lines  is  undoubtedly  due  to  the  very  high  temperature  of 


1 68 


ASTRONOMY. 


the  solar  atmosphere.  We  have  already  seen  that  the  lines  of 
the  spectrum  change  with  the  temperature ;  and,  as  the  tem- 
perature of  the  sun  is  far  higher  than  any  that  we  can  produce 
by  artificial  means,  we  might  reasonably  expect  that  it  would 
cause  the  disappearance  from  the  spectrum  of  many  lines 
which  we  find  to  be  present  at  our  highest  temperature. 

Lockyer  maintains  that  the  reason  why  no  trace  of  the  spec- 
tral lines  of  certain  of  our  so-called  elements  is  found  in  the 
solar  atmosphere  is,  that  these  substances  are  not  really  elemen- 
tary, and  that  the  intense  heat  of  the  sun  resolves  them  into 
simpler  constituents. 

MOTION  AT  THE  SURFACE  OF  THE  SUN. 

1 66.  Change  of  Pitch  caused  by  Motion  of  Sounding 
Body.  —  When  a  sounding  body  is  moving  rapidly  towards 


K9J 


Fig.  183. 

us,  the  pitch  of  its  note  becomes  somewhat  higher  than 
when  the  body  is  stationary ;  and,  when  such  a  body  is 
moving  rapidly  from  us,  the  pitch  of  its  note  is  lowered 
somewhat.  We  have  a  good  illustration  of  this  change  of 
pitch  at  a  country  railway  station  on  the  passage  of  an 
express-train.  The  pitch  of  the  locomotive  whistle  is  con- 
siderably higher  when  the  train  is  approaching  the  station 
than  when  it  is  leaving  it. 

167.  Explanation  of  the  Change  of  Pitch  produced  by 
Motion.  —  The  pitch  of  sound  depends  upon  the  rapidity 
with  which  the  pulsations  of  sound  beat  upon  the  drum  of 


ASTRONOMY. 


169 


the  ear.     The  more  rapidly  the  pulsations  follow  each  other, 

the  higher  is  the  pitch  :    hence 

the     shorter     the     sound-waves 

(provided  the  sound   is  all  the 

while    travelling     at     the    same 

rate),  the    higher   the   pitch    of 

the    sound.      Any   thing,    then, 

which  tends  to  shorten  the  waves 

of  sound  tends  also  to  raise  its 

pitch,  and  any  thing  which  tends 

to  lengthen  these  waves  tends  to 

lower  its  pitch. 

When  a  sounding  body  is  mov- 
ing rapidly  forward,  the  sound- 
waves are  crowded  together  a 
little,  and  therefore  shortened ; 
when  it  is  moving  backward,  the 
sound-waves  are  drawn  out,  or 
lengthened  a  little. 


The  effect  of  the  motion  of  a 
sounding  body  upon  the  length 
of  its  sonorous  waves  will  be 
readily  seen  from  the  following 
illustration  :  Suppose  a  number 
of  persons  stationed  at  equal 
intervals  in  a  line  on  a  long 
platform  capable  of  moving  back- 
ward and  forward.  Suppose  the 
men  are  four  feet  apart,  and  all 
walking  forward  at  the  same 
rate,  and  that  the  platform  is 
stationary,  and  that,  as  the  men 
leave  the  platform,  they  keep  on 
walking  at  the  same  rate  :  the  men 


Fig.  184. 


will   evidently  be   four   feet   apart    in   the  line  in  front  of  the 
platform,  as  well  as  on  it.     Suppose  next,  that  the  platform  is 


I/O  ASTRONOMY. 

moving  forward  at  the  rate  of  one  foot  in  the  interval  between 
two  men's  leaving  the  platform,  and  that  the  men  continue  to 
walk  as  before :  it  is  evident  that  the  men  will  then  be  three 
feet  apart  in  the  line  after  they  have  left  the  platform.  The 
forward  motion  of  the  platform  has  the  effect  of  crowding  the 
men  together  a  little.  Were  the  platform  moving  backward  at 
the  same  rate,  the  men  would  be  five  feet  apart  after  they  had 
left  the  platform.  The  backward  motion  of  the  platform  has 
the  effect  of  separating  the  men  from  one  another. 

The  distance  between  the  men  in  this  illustration  corre- 
sponds to  the  length  of  the  sound-wave,  or  the  distance  between 
its  two  ends.  Were  a  person  to  stand  beside  the  line,  and 
count  the  men  that  passed  him  in  the  three  cases  given  above, 
he  would  find  that  more  persons  would  pass  him  in  the  same 
time  when  the  platform  is  moving  forward  than  when  it  is 
stationary,  and  fewer  persons  would  pass  him  in  the  same  time 
when  the  platform  is  moving  backward  than  when  it  is  sta- 
tionary. In  the  same  way,  when  a  sounding  body  is  moving 
rapidly  forward,  the  sound-waves  beat  more  rapidly  upon  the 
ear  of  a  person  who  is  standing  still  than  when  the  body  is  at 
rest,  and  less  rapidly  when  the  sounding  body  is  moving  rapidly 
backward. 

Were  the  platform  stationary,  and  were  the  person  who  is 
counting  the  men  to  be  walking  along  the  line,  either  towards 
or  away  from  the  platform,  the  effect  upon  the  number  of  men 
passing  him  in  a  given  time  would  be  precisely  the  same  as  it 
would  be  were  the  person  stationary,  and  the  platform  moving 
either  towards  or  away  from  him  at  the  same  rate.  So  the 
change  in  the  rapidity  with  which  pulsations  of  sound  beat  upon 
the  ear  is  precisely  the  same  whether  the  ear  is  stationary  and 
the  sounding  body  moving,  or  the  sounding  body  is  stationary 
and  the  ear  moving. 

1 68.  Change  of  Refrangibility  due  to  the  Motion  of  a 
Luminous  Body.  —  Refrangibility  in  light  corresponds  to 
pitch  in  sound,  and  depends  upon  the  length  of  the  lumi- 
nous waves.-  The  shorter  the  luminous  waves,  the  greater 
the  refrangibility  of  the  waves.  Very  rapid  motion  of  a 
luminous  body  has  the  same  effect  upon  the  length  of  the 


ASTRONOMY.  I J I 

luminous  waves  that  motion  of  a  sounding  body  has  upon 
the  length  of  the  sonorous  waves.  When  a  luminous  body 
is  moving  very  rapidly  towards  us,  its  luminous  waves  are 
shortened  a  little,  and  its  light  becomes  a  little  more 
refrangible ;  when  the  luminous  body  is  moving  rapidly 
from  us,  its  luminous  waves  are  lengthened  a  little,  and  its 
light  becomes  a  little  less  refrangible. 

169.  Displacement  of  Spectral  Lines.  —  In  examining  the 
spectra  of  the  stars,  we  often  find  that  certain  of  the  dark 
lines  are  displaced  somewhat,  either  towards  the  red  or  the 
violet  end  of  the  spectrum.  As  the  dark  lines  are  in  the 
same  position  as  the  bright  lines  of  the  absorbing  vapor 
would  be,  a  displacement  of 
the  lines  towards  the  red  end 
of  the  spectrum  indicates  a 
lowering  of  the  refrangibility  of 
the  rays,  due  to  a  motion  of 
the  luminous  vapor  away  from 
us ;  and  a  displacement  of  the 
lines  towards  the  violet  end  of 
the  spectrum  indicates  an  in- 
crease of  refrangibility,  due  to  a 
motion  of  the  luminous  vapor 

towards  us.  From  the  amount  of  the  displacement  of  the 
lines,  it  is  possible  to  calculate,  the  velocity  at  which  the 
luminous  gas  is  moving.  In  Fig.  185  is  shown  the  dis- 
placement of  the  F  line  in  the  spectrum  of  Sirius.  This 
is  one  of  the  hydrogen  lines.  R  V  is  the  spectrum,  R 
being  the  red,  and  V  the  violet  end.  The  long  vertical 
line  is  the  bright  F  line  of  hydrogen,  and  the  short 
dark  line  to  the  left  of  it  is  the  position  of  the  F  line 
in  the  spectrum  of  Sirius.  It  is  seen  that  this  line  is  dis- 
placed somewhat  towards  the  red  end  of  the  spectrum. 
This  indicates  that  Sirius  must  be  moving  from  us ;  and 
the  amount  of  the  displacement  indicates  that  the  star 


1/2  ASTRONOMY. 

must  be   moving  at  the  rate  of  some  twenty-five  or  thirty 
miles  a  second. 

170.  Contortion  of  Lines  on  the  Disk  of  the  Sun. — 
Certain  of  the  dark  lines  seen  on  the  centre  of  the  sun's 
disk  often  appear  more  or  less  distorted,  as  shown  in 
Fig.  1 86,  which  represents  the  contortion  of  the  hydrogen 
line  as  seen  at  various  times,  i  and  2  indicate  a  rapid 
motion  of  hydrogen  away  from  us,  or  a  down-rush  at  the 
sun ;  3  and  4  (in  which  the  line  at  the  centre  is  dark  on 
one  side,  and  bent  towards  the  red  end  of  the  spectrum, 
and  bright  on  the  other  side  with  a  distortion  towards  the 
violet  end  of  the  spectrum)  indicate  a  down-rush  of  cool 
hydrogen  side  by  side  with  an  up-rush  of  hot  and  bright 


Fig.  186. 

hydrogen ;    5    indicates   local   down-rushes  associated  with 
quiescent  hydrogen. 

The  contorted  lines,  which  indicate  a  violently  agitated 
state  of  the  sun's  atmosphere,  appear  in  the  midst  of  other 
lines  which  indicate  a  quiescent  state.  This  is  owing  to 
the  fact  that  the  absorption  which  produces  the  dark  lines 
takes  place  at  various  depths  in  the  solar  atmosphere. 
There  may  be  violent  commotion  in  the  lower  layers  of  the 
sun's  atmosphere,  and  comparative  quiet  in  the  upper  layers. 
In  this  case,  the  lines  which  are  due  to  absorption  in  the 
lower  layers  would  indicate  this  disturbance  by  their  contor- 
tions ;  while  the  lines  produced  by  absorption  in  the  upper 
layers  would  be  free  from  contortion. 


ASTRONOMY. 


173 


It  often  happens,  too,  that  the  contortions  are  confined 
to  one  set  of  lines  of  an  element,  while  other  lines  of  the 
same  element  are  entirely  free  from  contortions.  This  is 
undoubtedly  due  to  the  fact  that  different  layers  of  the 
solar  atmosphere  differ  greatly  in  temperature ;  so  that  the 
same  element  would  give  one  set  of  lines  at  one  depth,  and 
another  set  at  another  depth :  hence  commotion  in  the 
solar  atmosphere  at  any  particular  depth  would,  be  indi- 
cated by  the  contortion  of  those  lines  of  the  element  only 
which  are  produced  by  the  temperature  at  that  particular 
depth. 

A  remarkable  case  of  contortion  witnessed  by  Professor 
Young  is  shown  in  Fig.  187.  Three  successive  appearances 

of    the    C    line    are  •••••^•^••••••••••••i 

shown.  The  second 
view  was  taken  three 
minutes  after  the  first, 
and  the  third  five 
minutes  after  the  sec- 
ond. The  contortion 
in  this  case  indicated 
a  velocity  ranging  Fig.  *87. 

from  two  hundred  to  three  hundred  miles  a  second. 

171.  Contortion  of  Lines  on  the  Sun's  Limb.  —  When 
the  spectroscope  is  directed  to  the  centre  of  the  sun's 
disk,  the  distortion  of  the  lines  indicates  only  vertical 
motion  in  the  sun's  atmosphere ;  but,  when  the  spectro- 
scope is  directed  to  the  limb  of  the  sun,  displacements  of 
the  lines  indicate  horizontal  motions  in  the  sun's  atmos- 
phere. When  a  powerful  spectroscope  is  directed  to  the 
margin  of  the  sun's  disk,  so  that  the  slit  of  the  collimator 
tube  shall  be  perpendicular  to  the  sun's  limb,  one  or  more 
of  the  dark  lines  on  the  disk  are  seen  to  be  prolonged  by 
a  bright  line,  as  shown  in  Fig.  188.  But  this  prolongation, 
instead  of  being  straight  and  narrow,  as  shown  in  the  figure, 


174 


ASTRONOMY. 


is  often  widened  and  distorted  in  various  ways,  as  shown  in 
Fig.  189.  In  the  left-hand  portion  of  the  diagram,  the  line 
is  deflected  towards  the  red  end  of  the  spectrum;  this 
indicates  a  violent  wind  on  the  sun's  surface  blowing  away 

from  us.  In  the  right- 
hand  portion  of  the  dia- 
gram, the  line  is  deflected 
towards  the  violet  end  of 
the  spectrum ;  this  indi- 
cates a  violent  wind  blow- 
ing towards  us.  In  the 
middle  portion  of  the 
figure,  the  line  is  seen  to 
be  bent  both  ways ;  this 
indicates  a  cyclone,  on 
one  side  of  which  the 
wind  would  be  blowing 


Fig.  188. 


from  us,  and  on  the  other  side  towards  us. 

The  distortions  of  the  solar  lines  indicate  that  the  wind 
at  the  surface  of  the  sun  often  blows  with  a  velocity  of 
from  one  hundred  to  three  liundred  miles  a  second.  The 


most  violent  wind  known  on  the   earth  has  a  velocity  of 
a  hundred  miles  an  hour. 


ASTRONOMY. 


175 


III.    THE   PHOTOSPHERE   AND   SUN   SPOTS. 

THE  PHOTOSPHERE. 

172.  The  Granulation  of  the  Photosphere.  —  When  the 
surface  of  the  sun  is  examined  with  a  good  telescope  under 
favorable  atmospheric  conditions,  it  is  seen  to  be  composed 


Fig.  190. 

of  minute  grains  of  intense  brilliancy  and  of  irregular  form, 
floating  in  a  darker  medium,  and  arranged  in  streaks  and 
groups,  as  shown  in  Fig.  190.  With  a  rather  low  power, 
the  general  effect  of  the  surface  is  much  like  that  of  rough 
drawing-paper,  or  of  curdled  milk  seen  from  a  little  dis- 
tance. With  a  high  power  and  excellent  atmospheric  con- 
ditions, the  grains  are  seen  to  be  irregular,  rounded  masses, 


1/6  ASTRONOMY. 

some  hundreds  of  miles  in  diameter,  sprinkled  upon  a  less 
brilliant  background,  and  appearing  somewhat  like  snow- 
flakes  sparsely  scattered  over  a  grayish  cloth.  Fig.  191  is 
a  representation  of  these  grains  according  to  Secchi. 

With  a  very  powerful  telescope  and  the  very  best  atmos- 
pheric conditions,  the  grains  themselves  are  resolved  into 
granules,  or  little  luminous  dots,  not  more  than  a  hundred 
miles  or  so  in  diameter,  which,  by  their  aggregation,  make 
up  the  grains,  just  as  they,  in  their  turn,  make  up  the  coarser 
masses  of  the  solar  surface.  Professor  Langley  estimates 
that  these  granules  constitute  about  one-fifth  of  the  sun's 

surface,  while  they 
emit  at  least  three- 
fourths  of  its  light. 

173.  Shape  of  the 
Grains.  —  The  grains 
differ  considerably  in 
shape  at  different 
times  and  on  differ- 
ent parts  of  the  sun's 
surface.  Nasmyth,  in 
1 86 1,  described  them 

Fig.  191.  as     willow-leaves     in 

shape,  several  thousand  miles  in  length,  but  narrow  and 
with  pointed  ends.  He  figured  the  surface  of  the  sun  as 
a  sort  of  basket-work  formed  by  the  interweaving  of  such 
filaments.  To  others  they  have  appeared  to  have  the  form 
of  rice-grains.  On  portions  of  the  sun's  disk  the  elemen- 
tary structure  is  often  composed  of  long,  narrow,  blunt- 
ended  filaments,  not  so  much  like  willow-leaves  as  like  bits 
of  straw  lying  roughly  parallel  to  each  other,  —  a  thatch- 
straw  formation,  as  it  has  been  called.  This  is  specially 
common  in  the  immediate  neighborhood  of  the  spots. 

174.  Nature  of  the  Grains. — The  grains  are,  undoubt- 
edly, incandescent  clouds  floating  in  the  sun's  atmosphere, 


ASTRONOMY.  177 

and  composed  of  partially  condensed  metallic  vapors,  just 
as  the  clouds  of  our  atmosphere  are  composed  of  partially 
condensed  aqueous  vapor.  Rain  on  the  sun  is  composed 
of  white-hot  drops  of  molten  iron  and  other  metals ;  and 
these  drops  are  often  driven  with  the  wind  with  a  velocity 
of  over  a  hundred  miles  a  second. 

As  to  the  forms  of  the  grains,  Professor  Young  says,  "  If 
one  were  to  speculate  as  to  the  explanation  of  the  grains 
and  thatch-straws,  it  might  be  that  the  grains  are  the  upper 
ends  of  long  filaments  of  luminous  cloud,  which,  over  most 
of  the  sun's  surface,  stand  approximately  vertical,  but  in 
the  neighborhood  of  a  spot  are  inclined  so  as  to  lie  nearly 
horizontal.  This  is  not  certain,  though  :  it  may  be  that  the 
cloud-masses  over  the  more  quiet  portions  of  the  solar  sur- 
face are  really,  as  they  seem,  nearly  globular,  while  near  the 
spots  they  are  drawn  out  into  filamentary  forms  by  atmos- 
pheric currents." 

175  Faculce.  —  The /acute  are  irregular  streaks  of  greater 
brightness  than  the  general  surface,  looking  much  like  the 
flecks  of  foam  on  the  surface  of  a  stream  below  a  water- 
fall. They  are  sometimes  from  five  to  twenty  thousand 
miles  in  length,  covering  areas  immensely  larger  than  a 
terrestrial  continent. 

These  faculae  are  elevated  regions  of  the  solar  surface, 
ridges  and  crests  of  luminous  matter,  which  rise  above  the 
general  level  of  the  sun's  surface,  and  protrude  through  the 
denser  portions  of  the  solar  atmosphere.  When  one  of 
these  passes  over  the  edge  of  the  sun's  disk,  it  can  be  seen 
to  project,  like  a  little  tooth.  Any  elevation  on  the  sun  to 
be  perceptible  at  all  must  measure  at  least  half  a  second 
of  an  arc,  or  two  hundred  and  twenty-five  miles. 

The  faculae  are  most  numerous  in  the  neighborhood 
of  the  spots,  and  much  more  conspicuous  near  the  limb 
of  the  sun  than  near  the  centre  of  the  disk.  Fig.  192  gives 
the  general  appearance  of  the  faculae,  and  the  darkening 


178  ASTRONOMY, 

of  the  limb  of  the  sun.  Near  the  spots,  the  faculae  often 
undergo  very  rapid  change  of  form,  while  elsewhere  on  the 
disk  they  change  rather  slowly,  sometimes  undergoing  little 
apparent  alteration  for  several  days. 

176.  Why  the  Faculcz  are  most  Conspicuous  near  the 
Limb  of  the  Sun.  —  The  reason  why  the  faculae  are  most 
conspicuous  near  the  limb  of  the  sun  is  this  :  The  luminous 
surface  of  the  sun  is  covered  with  an  atmosphere,  which, 
though  not  very  thick  compared  with  the  diameter  of  the 
sun,  is  still  sufficient  to  absorb  a  good  deal  of  light.  Light 


*'••-.% 


Fig.  192. 

coming  from  the  centre  of  the  sun's  disk  penetrates  this 
atmosphere  under  the  most  favorable  conditions,  and  is  but 
slightly  reduced  in  amount.  The  edges  of  the  disk,  on  the 
other  hand,  are  seen  through  a  much  greater  thickness  of 
atmosphere ;  and  the  light  is  reduced  by  absorption  some 
seventy-five  per  cent.  Suppose,  now,  a  facula  were  suf- 
ficiently elevated  to  penetrate  quite  through  this  atmosphere. 
Its  light  would  be  undimmed  by  absorption  on  any  part 
of  the  sun's  disk ;  but  at  the  centre  of  the  disk  it  would 
be  seen  against  a  background  nearly  as  bright  as  itself, 
while  at  the  margin  it  would  be  seen  against  one  only  a 


ASTRONOMY. 


179 


quarter  as  bright.  It  is  evident  that  the  light  of  any  facula, 
owing  to  the  elevation,  would  be  reduced  less  rapidly  as 
we  approach  the  edge  of  the  disk  than  that  of  the  general 
surface  of  the  sun,  which  lies  at  a  lower  level. 

SUN-SPOTS. 

177.  General  Appearance  of  Sun-Spots.  —  The  general 
appearance  of  a  well-formed  sun-spot  is  shown  in  Fig.  193. 
The  spot  consists  of  a  very  dark  central  portion  of  irregu- 


Fig.  193. 

lar  shape,  called  the  umbra,  which  is  surrounded  by  a 
less  dark  fringe,  called  the  penumbra.  The  penumbra  is 
made  up,  for  the  most  part,  of  filaments  directed  radially 
inward. 

There  is  great  variety  in  the  details  of  form  in  different 
sun-spots ;  but  they  are  generally  nearly  circular  during  the 
middle  period  of  their  existence.  During  the  period  of 
their  development  and  of  their  disappearance  they  are  much 
more  in  egular  in  form. 


1 8O  ASTRONOMY. 

There  is  nothing  like  a  gradual  shading-off  of  the  penum- 
bra, either  towards  the  umbra  on  the  one  side,  or  towards 
the  photosphere  on  the  other.  The  penumbra  is  separated 
from  both  the  umbra  and  the  photosphere  by  a  sharp  line 
of  demarcation.  The  umbra  is  much  brighter  on  the  inner 
than  on  the  outer  edge,  and  frequently  the  photosphere  is 
excessively  bright  at  the  margin  of  the  penumbra.  The 
brightness  of  the  inner  penumbra  seems  to  be  due  to  the 
crowding  together  of  the  penumbral  filaments  where  they 
overhang  the  edge  of  the  umbra. 

There  is  a  general  antithesis  between  the  irregularities  of 
the  outer  and  inner  edges  of  the  penumbra.  Where  an 
angle  of  the  penumbral  matter  crowds  in  upon  the  umbra, 
it  is  generally  matched  by  a  corresponding  outward  exten- 
sion into  the  photosphere,  and  vice  versa. 

The  umbra  of  the  spot  is  far  from  being  uniformly  dark. 
Many  of  the  penumbral  filaments  terminate  in  little  de- 
tached grains  of  luminous  matter  ;  and  there  are  also  faint- 
er veils  of  a  substance  less  brilliant,  but  sometimes  rose- 
colored,  which  seem  to  float  above  the  umbra.  The  umbra 
itself  is  made  up  of  masses  of  clouds  which  are  really 
intensely  brilliant,  and  which  appear  dark  only  by  contrast 
with  the  intenser  brightness  of  the  solar  surface.  Among 
these  clouds  are  often  seen  one  or  more  minute  circular 
spots  much  darker  than  the  rest  of  the  umbra.  These 
darker  portions  are  called  nuclei.  They  seem  to  be  the 
mouths  of  tubular  orifices  penetrating  to  unknown  depths. 
The  faint  veils  mentioned  above  continually  melt  away,  and 
are  replaced  by  others  in  some  different  position.  The 
bright  granules  at  the  tips  of  the  penumbral  filaments  seem 
to  sink  and  dissolve,  while  fresh  portions  break  off  to  replace 
them.  There  is  a  continual  indraught  of  luminous  matter 
over  the  whole  extent  of  the  penumbra. 

At  times,  though  very  rarely,  patches  of  intense  brightness 
suddenly  break  out,  remain  visible  for  a  few  minutes,  and 


ASTRONOMY.  l8l 

move  over  the  spot  with  velocities  as  great  as  a  hundred 
miles  a  second. 

The  spots  change  their  form  and  size  quite  percep- 
tibly from  day  to  day,  and  sometimes  even  from  hour  to 
hour. 

178.  Duration   of  Sun-Spots.  —  The   average   life   of  a 
sun-spot  is  two  or  three  months  :  the  longest  on  record  is 
that  of  a  spot  observed  in  1840  and  1841.  which  lasted 
eighteen  months.     There  are  cases,  however,  where  the  dis- 
appearance of  a  spot  is  very  soon  followed  by  the  appear- 
ance   of  another  at   the   same  point ;    and  sometimes  this 
alternate  disappearance  and  re-appearance  is  several  times 
repeated.     While    some    spots   are    thus   long-lived,    others 
endure   only   a   day   or   two,    and   sometimes   only   a   few 
hours. 

179.  Groups  of  Spots.  —  The   spots  usually  appear  not 
singly,  but  in  groups.     A  large  spot  is  often  followed  by  a 
train  of  smaller  ones  to  the  east  of  it,  many  of  which  are 
apt  to  be  irregular  in  form  and  very  imperfect  in  structure, 
sometimes  with  no  umbra  at  all,  often  with  a  penumbra  only 
on  one  side.     In  such  cases,  when  any  considerable  change 
of  form   or  structure  shows  itself  in  the  principal  spot,  it 
seems  to  rush  westward  over  the  solar  surface,  leaving  its 
attendants  trailing  behind.     When  a  large  spot  divides  into 
two  or  more,  as  often  happens,  the  parts  usually  seem  to 
repel  each  other,  and  fly  apart  with  great  velocity. 

1 80.  Size  of  the  Spots.  —  The    spots   are    sometimes    of 
enormous  size.     Groups  have  often  been  observed  covering 
areas  of  more  than  a  hundred  thousand  miles  square,  and 
single  spots  occasionally  measure  from  forty  to  fifty  thousand 
miles   in  diameter,    the   umbra  being   twenty-five   or  thirty 
thousand  miles  across.     A  spot,  however,   measuring  thirty 
thousand  miles  over  all,  may  be   considered   a  large  one. 
Such  a  spot  can  easily  be  seen  without  a  telescope  when 
the  brightness  of  the  sun's  surface  is  reduced  by  clouds  or 


1 82  ASTRONOMY. 

nearness  to  the  horizon,  or  by  the  use  of  colored  glass. 
During  the  years  1871  and  1872  spots  were  visible  to  the 
naked  eye  for  a  considerable  portion  of  the  time.  The 
largest  spot  yet  recorded  was  observed  in  1858.  It  had  a 
breadth  of  more  than  a  hundred  and  forty-three  thousand 
miles,  or  nearly  eighteen  times  the  diameter  of  the  earth, 
and  covered  about  a  thirty-sixth  of  the  whole  surface  of 
the  sun. 

Fig.   194  represents  a  group  of   sun-spots  observed   by 


Fig.  194. 


Professor  Langley,  and  drawn  on  the  same  scale  as  the  small 
circle  in  the  upper  left-hand  corner,  which  represents  the 
surface  of  half  of  our  globe. 

181.  The  Penumbral  Filaments.  —  Not  unfrequently  the 
penumbral  filaments  are  curved  spirally,  indicating  a  cyclonic 
action,  as  shown  in  Fig.  195.  In  such  cases  the  whole  spot 
usually  turns  slowly  around,  sometimes  completing  an  entire 
revolution  in  a  few  days.  More  frequently,  however,  the 
spiral  motion  lasts  but  a  short  time  ;  and  occasionally,  after 
continuing  for  a  while  in  one  direction,  the  motion  is 
reversed.  Very  often  in  large  spots  we  observe  opposite 


ASTRONOMY. 


1 83 


spiral  movements  in  different  portions   of  the   umbra,  as 
shown  in  Figs.  196  and  197. 


Fig.  195. 


Neighboring   spots   show  no   tendency  to  rotate  in  the 


Fig.  196. 


same  direction.     The  number  of  spots  in  which  a  decided 
cyclonic  motion   (like  that  shown  in  Fig.   198)  appears  is 


1 84  ASTRONOMY. 

comparatively  small,  not  exceeding  two  or  three  per  cent 
of  the  whole. 


Fig.  197. 

Plate  II.  represents  a  typical  sun-spot  as  delineated  by 


Fig. 

Professor  Langley.     At  the  left-hand  and  upper  portions  of 
this  great  spot  the  filaments  present  the  ordinary  appearance, 


„!, 


PLATE  E. 


ASTRONOMY.  185 

while  at  the  lower  edge,  and  upon  the  great  overhanging 
branch,  they  are  arranged  very  differently.  The  feathery 
brush  below  the  branch,  closely  resembling  a  frost-crystal 
on  a  window-pane,  is  as  rare  as  it  is  curious,  and  has  not 
been  satisfactorily  explained. 

182.  Birth  and  Decay  of  Sun- Spots.  —  The  formation  of 
a  spot  is  sometimes  gradual,  requiring  days  or  even  weeks 
for  its  full  development ;  and  sometimes  a  single  day  suf- 
fices. Generally,  for  some  time  before  its  appearance,  there 


Fig.  199. 

is  an  evident  disturbance  of  the  solar  surface,  indicated  espe- 
cially by  the  presence  of  many  brilliant  faculae,  among  which 
pores,  or  minute  black  dots,  are  scattered.  These  enlarge, 
and  between  them  appear  grayish  patches,  in  which  the 
photospheric  structure  is  unusually  evident,  as  if  they  were 
caused  by  a  dark  mass  lying  below  a  thin  veil  of  luminous 
filaments.  This  veil  seems  to  grow  gradually  thinner,  and 
finally  breaks  open,  giving  us  at  last  the  complete  spot  with 
its  penumbra.  Some  of  the  pores  coalesce  with  the  princi- 
pal spot,  some  disappear,  and  others  form  the  attendant 


1 86 


ASTRONOMY. 


train  before  described  (179).  The  spot  when  once  formed 
usually  assumes  a  circular  form,  and  remains  without  striking 
change  until  it  disappears.  As  its  end  approaches,  the 
surrounding  photosphere  seems  to  crowd  in,  and  overwhelm 
the  penumbra.  Bridges  of  light  (Fig.  199),  often  much 
brighter  than  the  average  of  the  solar  surface,  push  across 
the  umbra;  the  arrangement  of  the  penumbra  filaments 
becomes  confused ;  and,  as  Secchi  expresses  it,  the  lumi- 
nous matter  of  the  photosphere  seems  to  tumble  pell-mell 
into  the  chasm,  which  disappears,  and  leaves  a  disturbed 
surface  marked  with  faculae,  which,  in  their  turn,  gradually 
subside. 

183.  Motion  of  Sun-Spots .  —  The   spots    have   a  regular 

motion  across  the  disk  of  the  sun 
from  east  to  west,  occupying  about 
twelve  days  in  the  transit.  A  spot 
generally  appears  first  on  or  near 
the  east  limb,  and,  after  twelve  or 
fourteen  days,  disappears  at  the  west 
limb.  At  the  end  of  another  four- 
teen days,  or  more,  it  re-appears  at 
Fi§-  20°-  the  east  limb,  unless,  in  the  mean 

time,  it  has  vanished  from  sight  entirely.  This  motion  of 
the  spots  is  indicated  by  the  arrow  in  Fig.  200.  The 
interval  between  two  successive  appearances  of  the  same 
spot  on  the  eastern  edge  of  the  sun  is  about  twenty-seven 
days. 

184.  The  Rotation  of  the  Sun. — The  spots  are  evidently 
carried  around  by  the  rotation  of  the  sun  on  its  axis.     It  is 
evident,  from  Fig.  201,  that  the  sun  will  need  to  make  more 
than  a  complete  rotation   in  order  to   bring  a  spot  again 
upon  the   same   part  of  the  disk  as  seen  from   the   earth. 
6*  represents  the  sun,  and  E  the  earth.     The  arrows  indicate 
the  direction  of  the  sun's  rotation.     When  the  earth  is  at  E, 
a  spot  at  a  would  be  seen  at  the  centre  of  the  solar  disk. 


ASTRONOMY. 


1 87 


While  the  sun  is  turning  on  its  axis,  the  earth  moves  in  its 
orbit  from  E  to  E' :  hence  the  sun  must  make  a  complete 
rotation,  and  turn  from  a  to  a'  in  addition,  in  order  to 
bring  the  spot  again  to  the  centre  of  the  disk.  To  carry 
the  spot  entirely  around,  and 
then  on  to  a ',  requires  about 
twenty-seven  days.  From  this 
synodical  period  of  the  spot, 
as  it  might  be  called,  it  has 
been  calculated  that  the  sun 
must  rotate  on  its  axis  in 
about  twenty-five  days. 

185.  The  Inclination  of  the 
Sun 's  Axis.  —  The  paths  de- 
scribed by  sun-spots  across 
the  solar  disk  vary  with  the 
position  of  the  earth  in  its 
orbit,  as  shown  in  Fig.  202. 
We  therefore  conclude  that  Fis-  2CI- 

the  sun's  axis  is  not  perpendicular  to  the  plane  of  the 
earth's  orbit.  The  sun  rotates  on  its  axis  from  west  to  east, 
and  the  axis  leans  about  seven  degrees  from  the  perpendicu- 
lar to  the  earth's  orbit. 


Fig.  202. 

186.  The  Proper  Motion  of  the  Spots.  —  When  the 
period  of  the  sun's  rotation  is  deduced  from  the  motion  of 
spots  in  different  solar  latitudes,  there  is  found  to  be  con- 
siderable variation  in  the  results  obtained.  Thus  spots  near 


i88 


ASTRONOMY. 


the  equator  indicate  that  the  sun  rotates  in  about  twenty-five 
days;  while  those  in  latitude  20°  indicate  a  period  about 

eighteen  hours 
longer;  and  those 
in  latitude  30°  a 
period  of  twenty- 
seven  days  and  a 
half.  Strictly  speak- 
ing, the  sun,  as  a 
whole,  has  no  sin- 
gle period  of  rota- 
tion;  but  different 
portions  of  its  sur- 
face perform  their 
revolutions  in  dif- 
ferent times.  The 
equatorial  regions 

not  only  move  more  rapidly  in  miles  per  hour  than  the 
rest  of  the  solar  surface,  but  they  complete  the  entire  rota- 
tion in  shorter  time. 

There  appears  to 
be  a  peculiar  surface- 
drift  in  the  equatorial 
regions  of  the  sun, 
the  cause  of  which 
is  unknown,  but  which 
gives  the  spots  a 
proper  motion ;  that 
is,  a  motion  of  their 
own,  independent  of 
the  rotation  of  the 
sun. 

187.  Distribution  of  the  Sun- Spots. — The  sun-spots  are 
not  distributed  uniformly  over  the  sun's  surface,  but  occur 
mainly  in  two  zones  on  each  side  of  the  equator,  and  be- 


ASTRONOMY. 


189 


tween  the  latitudes  of  10°  and  30°,  as  shown  in  Fig.  203. 
On  and  near  the  equator  itself  they  are  comparatively  rare. 
There  are  still  fewer  beyond  35°  of  latitude,  and  only  a 
single  spot  has  ever  been  recorded  more  than  45°  from  the 
solar  equator. 

Fig.  204  shows  the  distribution  of 
the  sun-spots  observed  by  Carrington 
during  a  period  of  eight  years.  The 
irregular  line  on  the  left-hand  side  of 
the  figure  indicates  by  its  height  the 
comparative  frequency  with  which  the 
spots  occurred  in  different  latitudes. 
In  Fig.  205  the  same  thing  is  indicated 
by  different  degrees  of  darkness  in  the 
shading  of  the  belts. 

1 88.  The  Periodicity  of  the  Spots.— 
Careful  observations  of  the  solar  spots 
indicate  a  period  of  about  eleven  years 
in  the  spot-producing  activity  of  the 
sun.  During  two  or  three  years  the 
spots  increase  in  number  and  in  size ; 
then  they  begin  to  diminish,  and  reach 
a  minimum  five  or  six  years  after  the 
maximum.  Another  period  of  about 
six  years  brings  the  return  of  the  maxi- 
mum. The  intervals  are,  however, 
somewhat  irregular. 

Fig.  206  gives  a  graphic  representa- 
tion of  the  periodicity  of  the  sun-spots. 
The  height  of  the  curve  shows  the 
frequency  of  the  sun-spots  in  the  years  Ffe-  205- 

given  at  the  bottom  of  the  figure.  It  appears,  from  an 
examination  of  this  sun-spot  curve,  that  the  average  inter- 
val from  a  minimum  to  the  next  following  maximum  is 
only  about  four  years  and  a  half,  while  that  from  a  maxi- 


I9O  ASTRONOMY. 

mum  to  the  next  following  minimum  is  six  years  and  six- 
tenths.     The  disturbance  which  produces  the  sun-spots  is 

developed  suddenly,  but  dies  away 

gradually. 

189.  Connection  between  Sun- 
Spots  and  Terrestrial  Magnetism. 
—  The  magnetic  needle  does  not 
point  steadily  in  the  same  direction, 
but  is  subject  to  various  disturb- 
ances, some  of  which  are  regular, 
and  others  irregular. 

(i)  One  of  the  most  noticeable 
of  the  regular  magnetic  changes  is 
the  so-called  diurnal  oscillation. 
During  the  early  part  of  the  day 
the  north  pole  of  the  needle  moves 
toward  the  west  in  our  latitude, 
returning  to  its  mean  position  about 
ten  .P.M.,  and  remaining  nearly 
stationary  during  the  night.  The 
extent  of  this  oscillation  in  the 
United  States  is  about  fifteen  min- 
utes of  arc  in  summer,  and  not 
quite  half  as  much  in  winter;  but 
it  differs  very  much  in  different 
localities  and  at  different  times,  and 
the  average  diurnal  oscillation  in 
any  locality  increases  and  decreases 
pretty  regularly  during  a  period  of 
about  eleven  years.  The  maximum 
and  minimum  of  this  period  of 
magnetic  disturbance  are  found  to 
Flg-  2o6>  coincide  with  the  maximum  and 

minimum  of  the  sun-spot  period.     This  is  shown  in  Fig.  206, 

in  which  the  dotted  lines  indicate  the  variations  in  the  intensity 

of  the  magnetic  disturbance. 

(2)  Occasionally    so-called    magnetic    storms    occur,    during 

which    the   compass-needle    is    sometimes   violently  disturbed, 


ASTRONOMY. 


IQI 


oscillating  five  degrees,  or  even  ten  degrees,  within  an  hour 
or  two.  These  storms  are  generally  accompanied  by  an  aurora, 
and  an  aurora  is  always  accompanied  by  magnetic  disturbance. 
A  careful  comparison  of  aurora  observations  with  those  of  sun- 
spots  shows  an  almost  perfect  parallelism  between  the  curves 
of  auroral  and  sun-spot  frequency. 


Fig.  207. 

(3)  A  number  of  observations  render  it  very  probable  that 
every  intense  disturbance  of  the  solar  surface  is  propagated 
to  our  terrestrial  magnetism  with  the  speed  of  light. 

Fig.  207  shows  certain  of  the  solar  lines  as  they  were 
observed  by  Professor  Young  on  Aug.  3,  1872.  The  contor- 
tions of  the  F  line  indicated  an  intense  disturbance  in  the 


Fig.  208. 

atmosphere  of  the  sun.  There  were  three  especially  notable 
paroxysms  in  this  distortion,  occurring  at  a  quarter  of  nine, 
half-past  ten,  and  ten  minutes  of  twelve,  A.M. 

Fig.  208  shows  the  curve  of  magnetic  disturbance  as  traced 
at  Greenwich  on  the  same  day.  It  will  be  seen  from  the  curve 
that  it  was  a  day  of  general  magnetic  disturbance.  At  the 


192  ASTRONOMY. 

times  of  the  three  paroxysms,  which  are  given  at  the  bottom 
of  the  figure,  it  will  be  observed  that  there  is  a  peculiar  shiver- 
ing of  the  magnetic  curve. 


190.  The  Spots  are  Depressions  in  the  Photosphere. — 
This  fact  was  first  clearly  brought  out  by  Dr.  Wilson  of 
Glasgow,  in  1 769,  from  observations  upon  the  penumbra  of 
a  spot  in  November  of  that  year.  He  found,  that  when 
the  spot  appeared  at  the  eastern  limb,  or  edge  of  the  sun, 
just  moving  into  sight,  the  penumbra  was  well  marked  on 
the  side  of  the  spot  nearest  to  the  edge  of  the  disk ;  while 
on  the  other  edge  of  the  spot,  towards  the  centre  of  the 

sun,  there  was  no  penumbra 
visible  at  all,  and  the  umbra 
itself  was  almost  hidden,  as  if 
behind  a  bank.  When  the  spot 
had  moved  a  day's  journey 
toward  the  centre  of  the  disk, 
the  whole  of  the  umbra  came 
into  sight,  and  the  penumbra  on 
the  inner  edge  of  the  spot 
began  to  be  visible  as  a  narrow 
line.  After  the  spot  was  well 

advanced  upon  the  disk,  the  penumbra  was  of  the  same 
width  all  around  the  spot.  When  the  spot  approached  the 
sun's  western  limb,  the  same  phenomena  were  repeated,  but 
in  the  inverse  order.  The  penumbra  on  the  inner  edge  of 
the  spot  narrowed  much  faster  than  that  on  the  outer,  dis- 
appeared entirely,  and  finally  seemed  to  hide  from  sight 
much  of  the  umbra  nearly  a  whole  day  before  the  spot 
passed  from  view  around  the  limb.  This  is  precisely  what 
would  occur  (as  Fig.  209  clearly  shows)  if  the  spot  were  a 
saucer-shaped  depression  in  the  solar  surface,  the  bottom 
of  the  saucer  corresponding  to  the  umbra,  and  the  sloping 
sides  to  the  penumbra. 


ASTRONOMY. 


193 


191.  Sun-Spot  Spectrum.  —  When  the  image  of  a  sun-spot 
is  thrown  upon  the  slit  of  the  spectroscope,  the  spectrum  is 
seen  to  be  crossed  longitudinally  by  a  continuous  dark  band, 
showing  an  increased  general  absorption  in  the  region  of  the 
sun-spot.  Many  of  the  spectral  lines  are  greatly  thickened,  as 


Fig.  210. 

shown  in  Fig.  210.  This  thickening  of  the  lines  shows  that 
the  absorption  is  taking  place  at  a  greater  depth.  New  lines 
and  shadings  often  appear,  which  indicate,  that,  in  the  cooler 
nucleus  of  the  spot,  certain  compound  vapors  exist,  which  are 


Fig.  211. 

dissociated  elsewhere  on  the  sun's  surface.     These  lines  and 
shadings  are  shown  in  Fig.  211. 

It  often  happens  that  certain  of  the  spectral  lines  are  re- 
versed in  the  spectrum  of  the  spot,  a  thin  bright  line  appear- 
ing over  the  centre  of  a  thick  dark  one,  as  shown  in  Fig.  212. 
These  reversals  are  due  to  very  bright  vapors  floating  over  the 
spot. 


194 


ASTRONOMY. 


Fig.  212. 


At   times,  also,  the   spectrum   of  a   spot  indicates  violent 

motion  in  the  overlying  gases  by  distortion  and  displacement 

of  the  lines.     This  phenomenon  occurs  oftener  at  points  near 

the  outer  edge  of  the  penum- 
bra than  over  the  centre  of 
the  spot ;  but  occasionally  the 
whole  neighborhood  is  vio- 
lently agitated.  In  such  cases, 
lines  in  the  spectrum  side  by 
side  are  often  affected  in  en- 
tirely different  ways,  one  being 
greatly  displaced  while  its 
neighbor  is  not  disturbed  in 
the  least,  showing  that  the 

vapors  which  produce  the  lines  are  at  different  levels  in  the 

solar  atmosphere,  and  moving  independently  of  each  other. 
192.  The  Cause  and  Nature  of  Sun-Spots.  —  According  to 

Professor   Young, 

the      arrangement 

and    relations     of 

the     photospheric 

clouds       in       the 

neighborhood      of 

a  spot  are  such  as 

are  represented  in 

Fig.  213.      "  Over 

the   sun's   surface 

generally,       these 

clouds       probably 

have  the  form  of 

vertical     columns, 

as    at    a  a.      Just 

outside    the    spot, 

the    level    of    the 

photosphere         is 

the    most    part,   overtopped    by   eruptions   of    hydrogen   and 

usually  raised  into  faculae,  as  at  bb.     These  faculae  are,  for 

metallic  vapors,  as  indicated  by  the  shaded  clouds.  .  .  .  While 

the  great  clouds  of  hydrogen  are  found  everywhere  upon  the 


213. 


ASTRONOMY.  IQ5 

sun,  these  spiky,  vivid  outbursts  of  metallic  vapors  seldom 
occur  except  just  in  the  neighborhood  of  a  spot,  and  then  only 
during  its  season  of  rapid  change.  In  the  penumbra  of  the 
spot  the  photospheric  filaments  become  more  or  less  nearly 
horizontal,  as  at  pp;  in  the  umbra  at  u  it  is  quite  uncertain 
what  the  true  state  of  affairs  may  be.  We  have  conjecturally 
represented  the  filaments  there  as  vertical  also,  but  depressed 
and  carried  down  by  a  descending  current.  Of  course,  the 
cavity  is  filled  by  the  gases  which  overlie  the  photosphere ;  and 
it  is  easy  to  see,  that,  looked  at  from  above,  such  a  cavity 
and  arrangement  of  the  luminous  filaments  would  present  the 
appearances  actually  observed." 

Professor  Young  also  suggests  that  the  spots  may  be  depres- 
sions in  the  photosphere  caused  "by  the  diminution  of  upward 
pressure  from  below,  in  consequence  of  eruptions  in  the  neigh- 
borhood ;  the  spots  thus  being,  so  to  speak,  sinks  in  the 
photosphere.  Undoubtedly  the  photosphere  is  not  a  strictly 
continuous  shell  or  crust;  but  it  is  heavy  as  compared  with  the 
uncondensed  vapors  in  which  it  lies,  just  as  a  rain-cloud  in  our 
terrestrial  atmosphere  is  heavier  than  the  air;  and  it  is  proba- 
bly continuous  enough  to  have  its  upper  level  affected  by  any 
diminution  of  pressure  below.  The  gaseous  mass  below  the 
photosphere  supports  its  weight  and  the  weight  of  the  products 
of  condensation,  which  must  always  be  descending  in  an  incon- 
ceivable rain  and  snow  of  molten  and  crystallized  material. 
To  all  intents  and  purposes,  though  nothing  but  a  layer  of 
clouds,  the  photosphere  thus  forms  a  constricting  shell,  and 
the  gases  beneath  are  imprisoned  and  compressed.  Moreover, 
at  a  high  temperature  the  viscosity  of  gases  is  vastly  increased, 
so  that  quite  probably  the  matter  of  the  solar  nucleus  resem- 
bles pitch  or  tar  in  its  consistency  more  than  what  we  usually 
think  of  as  a  gas.  Consequently,  any  sudden  diminution  of 
pressure  would  propagate  itself  slowly  from  the  point  where 
it  occurred.  Putting  these  things  together,  it  would  seem,  that, 
whenever  a  free  outlet  is  obtained  through  the  photosphere  at 
any  point,  thus  decreasing  the  inward  pressure,  the  result  would 
be  the  sinking  of  a  portion  of  the  photosphere  somewhere  in 
the  immediate  neighborhood,  to  restore  the  equilibrium ;  and,  if 
the  eruption  were  kept  up  for  any  length  of  time,  the  depres- 


196  ASTRONOMY. 

sion  in  the  photosphere  would  continue  till  the  eruption  ceased. 
This  depression,  filled  with  the  overlying  gases,  would  constitute 
a  spot.  Moreover,  the  line  of  fracture  (if  we  may  call  it  so)  at 
the  edges  of  the  sink  would  be  a  region  of  weakness  in  the 
photosphere,  so  that  we  should  expect  a  series  of  eruptions 
all  around  the  spot.  For  a  time  the  disturbance,  therefore, 
would  grow,  and  the  spot  would  enlarge  and  deepen,  until,  in 
spite  of  the  viscosity  of  the  internal  gases,  the  equilibrium  of 
pressure  was  gradually  restored  beneath.  So  far  as  we  know 
the  spectroscopic  and  visual  phenomena,  none  of  them  con- 
tradict this  hypothesis.  There  is  nothing  in  it,  however,  to 
account  for' the  distribution  of  the  spots  in  solar  latitudes,  nor 
for  their  periodicity." 

i 
IV.     THE    CHROMOSPHERE    AND    PROMINENCES. 

193.  The  Sun's   Outer  Atmosphere.  —  What  we   see   of 
the  sun  under  ordinary  circumstances  is  but  a  fraction  of 
his   total   bulk.     While    by  far  the   greater  portion    of  the 
solar  mass   is   included  within  the  photosphere,  the  larger 
portion  of  his  volume  lies  without,  and  constitutes  a  gaseous 
envelope  whose  diameter  is  at  least  double,  and  its  bulk 
therefore  sevenfold,  that  of  the  central  globe. 

This  outer  envelope,  though  mainly  gaseous,  is  not  spheri- 
cal, but  has  an  exceedingly  irregular  and  variable  outline. 
It  seems  to  be  made  up,  not  of  regular  strata  of  different 
density,  like  our  atmosphere,  but  rather  of  flames,  beams, 
and  streamers,  as  transient  and  unstable  as  those  of  the 
aurora  borealis.  It  is  divided  into  two  portions  by  a 
boundary  as  definite,  though  not  so  regular,  as  that  which 
separates  them  both  from  the  photosphere.  The  outer  and 
far  more  extensive  portion,  which  in  texture  and  rarity  seems 
to  resemble  the  tails  of  comets,  is  known  as  the  coronal 
atmosphere,  since  to  it  is  chiefly  due  the  corona,  or  glory, 
which  surrounds  the  darkened  sun  during  an  eclipse. 

194.  The   Chromosphere.  —  At  the  base   of  the  coronal 
atmosphere,  and  in  contact  with  the  photosphere,  is  what 


ASTRONOMY. 


197 


resembles  a  sheet  of  scarlet  fire.  It  appears  as  if  countless 
jets  of  heated  gas  were  issuing  through  vents  over  the 
whole  surface,  clothing  it  with  flame,  which  heaves  and 
tosses  like  the  blaze  of  a  conflagration.  This  is  the  chro- 
mosphere',  or  color-sphere.  It  owes  its  vivid  redness  to  the 
predominance  of  hydrogen  in  the  flames.  The  average 
depth  of  the  chromosphere  is  not  far  from  ten  or  twelve 
seconds,  or  fitfe  thousand  or  six  thousand  miles. 

195.  The  Prominences.  —  Here  and  there  masses  of  this 
hydrogen,  mixed  with  other  substances,  rise  far  above  the 
general  level  into  the  coronal  regions,  where  they  float  like 
clouds,  or  are  torn 
to  pieces  by  conflict- 
ing currents.  These 
cloud  -  masses  are 
known  as  solar  promi- 
nences, or  protuber- 
ances. 

196.  Magnitude  and 
Distribution  of  the 
Prominences.  —  The 
prominences  differ 
greatly  in  magnitude. 
Of  the  2,767  observed  Fig' 2I4> 

by  Secchi,  1,964  attained  an  altitude  of  eighteen  thousand 
miles;  751,  or  nearly  a  fourth  of  the  whole,  reached  a 
height  of  twenty-eight  thousand  miles  j  several  exceeded 
eighty-four  thousand  miles.  In  rare  instances  they  reach 
elevations  as  great  as  a  hundred  thousand  miles.  A  few 
have  been  seen  which  exceeded  a  hundred  and  fifty  thou- 
sand miles ;  and  Secchi  has  recorded  one  of  three  hundred 
thousand  miles. 

The  irregular  lines  on  the  right-hand  side  of  Fig.  214 
show  the  proportion  of  the  prominences  observed  by  Secchi, 
that  were  seen  in  different  parts  of  the  sun's  surface.  The 


198  ASTRONOMY, 

outer  line  shows  the  distribution  of  the  smaller  prominences, 
and  the  inner  dotted  line  that  of  the  larger  prominences. 
By  comparing  these  lines  with  those 
on  the  opposite  side  of  the  circle, 
which  show  the  distribution  of  the 
spots,  it  will  be  seen,  that,  while  the 
spots  are  confined  mainly  to  two 
belts,  the  prominences  are  seen  in  all 
latitudes. 

197.  The  Spectrum  of  the  Chro- 
mosphere. —  The  spectrum  of  the 
chromosphere  is  comparatively  simple. 
There  are  eleven  lines  only  which  are 
always  present;  and  six  of  these  are 
lines  of  hydrogen,  and  the  others, 
with  a  single  exception,  are  of  un- 
known elements.  There  are  sixteen 
other  lines  which  make  their  appear- 
ance very  frequently.  Among  these 
latter  are  lines  of  sodium,  magnesium, 
and  iron. 

Where  some  special  disturbance  is 
going  on,  the  spectrum  at  the  base  of 
the  chromosphere  is  very  complicated, 
consisting  of  hundreds  of  bright  lines. 
"The  majority  of  the  lines,  however, 
are  seen  only  occasionally,  for  a  few 
minutes  at  a  time,  when  the  gases 
and  vapors,  which  generally  lie  low 
(mainly  in  the  interstices  of  the 
clouds  which  constitute  the  photo- 
sphere), and  below  its  upper  surface, 

are  elevated  for  the  time  being  by  some  eruptive  action. 
For  the  most  part,  the  lines  which  appear  only  at  such 
times  are  simply  reversals  of  the  more  prominent  dark  lines 


ASTRONOMY.  IQ9 

of  the  ordinary  solar  spectrum.  But  the  selection  of  the 
lines  seems  most  capricious :  one  is  taken,  and  another  left, 
though  belonging  to  the  same  element,  of  equal  intensity, 
and  close  beside  the  first."  Some  of  the  main  lines  of  the 
chromosphere  and  prominences  are  shown  in  Fig.  215. 

198.  Method  of  Studying  the  Chromosphere  and  Promi- 
nences. —  Until  recently,  the  solar  atmosphere  could  be  seen 
only  during  a  total  eclipse  of  the  sun;  but  now  the  spectro- 
scope enables  us  to  study  the  chromosphere  and  the  promi- 
nences with  nearly  the  same  facility  as  the  spots  and  faculae. 

The  protuberances  are  ordinarily  invisible,  for  the  same 
reason  that  the  stars  cannot  be  seen  in  the  daytime ;  they  are 
hidden  by  the  intense  light  reflected  from  our  own  atmosphere. 
If  we  could  only  get  rid  of  this  aerial  illumination,  without  at 
the  same  time  weakening  the  light  of 
the  prominences,  the  latter  would  be- 
come visible.  This  the  spectroscope 
enables  us  to  accomplish.  Since  the 
air-light  is  reflected  sunshine,  it  of 
course  presents  the  same  spectrum  as 
sunlight,  —  a  continuous  band  of  color 
crossed  by  dark  lines.  Now,  this  sort 

of  spectrum  is  weakened  by  increase  of  dispersive  power  (159), 
because  the  light  is  spread  out  into  a  longer  ribbon,  and  made 
to  cover  a  greater  area.  On  the  other  hand,  the  spectrum  of 
the  prominences,  being  composed  of  bright  lines,  undergoes 
no  such  diminution  by  increased  dispersion. 

When  the  spectroscope  is  used  as  a  means  of  examining  the 
prominences,  the  slit  is  more  or  less  widened.  The  telescope 
is  directed  so  that  the  image  of  that  portion  of  the  solar  limb 
which  is  to  be  examined  shall  be  tangent  to  the  opened  slit, 
as  in  Fig.  216,  which  represents  the  slit-plate  of  the  spectro- 
scope of  its  actual  size,  with  the  image  of  the  sun  in  the 
proper  position  for  observation. 

If,  now,  a  prominence  exists  at  this  part  of  the  solar  limb, 
and  if  the  spectroscope  itself  is  so  adjusted  that  the  C  line 
falls  in  the  centre  of  the  field  of  view,  then  one  will  see  some- 
thing like  Fig.  217.  "The  red  portion  of  the  spectrum  will 


2OO  ASTRONOMY. 

stretch  athwart  the  field  of  view  like  a  scarlet  ribbon  with  a 
darkish  band  across  it;  and  in  that  band  will  appear  the  promi- 
nences, like  scarlet  clouds,  so  like  our  own  terrestrial  clouds, 
indeed,  in  form  and  texture,  that  the  resemblance  is  quite 
startling.  One  might  almost  think  he  was  looking  out  through 
a  partly-opened  door  upon  a  sunset  sky,  except  that  there  is 
no  variety  or  contrast  of  color;  all  the  cloudlets  are  of  the 
same  pure  scarlet  hue.  Along  the  edge  of  the  opening  is  seen 
the  chromosphere,  more  brilliant  than  the  clouds  which  rise 
from  it  or  float  above  it,  and,  for  the  most  part,  made  up  of 
minute  tongues  and  filaments." 

199.   Quiescent  Prominences.  —  The   prominences   differ 

as  widely  in  form 
and  structure  as  in 
magnitude.  The  two 
principal  classes  are 
the  quiescent,  cloud- 
formed,  or  hydrogen- 
ous, and  the  eruptive, 
or  metallic. 

The          quiescent 
prominences    resem- 
ble    almost     exactly 

our  terrestrial  clouds,  and  differ  among  themselves  in  the 
same  manner.  They  are  often  of  enormous  dimensions, 
especially  in  horizontal  extent,  and  are  comparatively  per- 
manent, often  undergoing  little  change  for  hours  and  days. 
Near  the  poles  they  sometimes  remain  during  a  whole  solar 
revolution  of  twenty-seven  days.  Sometimes  they  appear 
to  lie  upon  the  limb  of  the  sun,  like  a  bank  of  clouds  in 
the  terrestrial  horizon,  probably  because  they  are  so  far 
from  the  edge  that  only  their  upper  portions  are  in  sight. 
When  fully  seen,  they  are  usually  connected  to  the  chromo- 
sphere by  slender  columns,  generally  smallest  at  the  base, 
and  often  apparently  made  up  of  separate  filaments  closely 


PLATE  Itt. 


ASTRONOMY. 


2OI 


intertwined,  and  expanding  upward.  Sometimes  the  whole 
under  surface  is  fringed  with  pendent  filaments.  Sometimes 
they  float  entirely 
free  from  the  chro- 
mosphere ;  and  in 
most  cases  the 
larger  clouds  are 
attended  by  de- 
tached cloudlets. 
Various  forms  of 
quiescent  promi- 
nences are  shown 
in  Plate  III. 
Other  forms  are 
given  in  Figs.  218 
and  219. 

Their     spectrum  Fig.  218. 

is  usually  very  simple/ consisting  of  the  four  lines  of  hydro- 
gen and  the  orange 
D^ :  hence  the  ap- 
pellation hydroge- 
nous. Occasionally 
the  sodium  and 
magnesium  lines 
also  appear,  even 
near  the  tops  of 
the  clouds. 

200.  Eruptive 
Prominences. — The 
eruptive  promi- 
nences ordinarily 

consist   of    brilliant 

Fig-  2I9-  spikes  or  jets,  which 

change  very  rapidly  in  form  and  brightness.     As  a  rule,  their 
altitude  is  not  more  than  twenty  thousand  or  thirty  thousand 


2O2  .  ASTRONOMY. 

miles ;  but  occasionally  they  rise  far  higher  than  even  the 
largest  of  the  quiescent  protuberances.  Their  spectrum  is 
very  complicated,  especially  near  their  base,  and  often  filled 
with  bright  lines.  The  most  conspicuous  lines  are  those  of 
sodium,  magnesium,  barium,  iron,  and  titanium :  hence 
Secchi  calls  them  metallic  prominences. 

They  usually  appear  in  the  immediate  vicinity  of  a  spot, 
never  very  near  the  solar  poles.  They  change  with  such 
rapidity,  that  the  motion  can  almost  be  seen  with  the  eye. 


Fig.  220. 

Sometimes,  in  the  course  of  fifteen  or  twenty  minutes,  a 
mass  of  these  flames,  fifty  thousand  miles  high,  will  undergo 
a  total  transformation  ;  and  in  some  instances  their  com- 
plete development  or  disappearance  takes  no  longer  time. 
Sometimes  they  consist  of  pointed  rays,  diverging  in  all 
directions,  as  represented  in  Fig.  220.  "Sometimes  they 
look  like  flames,  sometimes  like  sheaves  of  grain,  some- 
times like  whirling  water-spouts  capped  with  a  great  cloud ; 
occasionally  they  present  most  exactly  the  appearance  of 
jets  of  liquid  fire,  rising  and  falling  in  graceful  parabolas ; 
frequently  they  carry  on  their  edges  spirals  like  the  volutes 


ASTRONOMY. 


203 


Fig.    221. 


of  an  Ionic  column ;  and  continually  they  detach  fila- 
ments, which  rise  to  a  great  elevation,  gradually  expand- 
ing and  growing  fainter  as  they  ascend,  until  the  eye  loses 
them." 

20 1.  Change  of 
Form  in  Prominences. 
—  Fig.  221  represents 
a  prominence  as 
seen  by  Professor 
Young,  Sept.  7,  1871. 
It  was  an  immense 
quiescent  cloud,  a 
hundred  thousand  miles  long  and  fifty-four  thousand  miles 
high.  At  a  there  was  a  brilliant  lump,  somewhat  in  the 
form  of  a  thunder-head.  On  returning  to  the  spectro- 
scope less  than  half  an  hour 
afterwards,  he  found  that  the 
cloud  had  been  literally  blown 
into  shreds  by  some  incon- 
ceivable uprush  from  beneath. 
The  prominence  then  pre- 
sented the  form  shown  in 
Fig.  222.  The  debris  of  the 
cloud  had  already  attained  a 
height  of  a  hundred  thou- 
sand miles.  While  he  was 
watching  them  for  the  next 
ten  minutes,  they  rose,  with 
a  motion  almost  perceptible 
to  the  eye,  till  the  upper- 
most reached  an  altitude  of 

two  hundred  thousand  miles.  As  the  filaments  rose,  they 
gradually  faded  away  like  a  dissolving  cloud. 

Meanwhile  the  little  thunder-head  had  grown  and  devel- 
oped into  what  appeared  to  be  a  mass  of  rolling  and  ever- 


2O4  ASTRONOMY. 

changing  flame.     Figs.   223  and   224  give  the  appearance 


FiS-  223-  Fig.  224. 

of  this    portion  of  the  prominence   at   intervals  of  fifteen 
minutes.     Other  similar  eruptions  have  been  observed. 

V.     THE   CORONA. 

202.  General  Appearance  of  the  Corona.  —  At  the 
time  of  a  total  eclipse  of  the  sun,  if  the  sky  is  clear,  the 
moon  appears  as  a  huge  black  ball,  the  illumination  at 
the  edge  .of  the  disk  being  just  sufficient  to  bring  out 
its  rotundity.  "  From  behind  it,"  to  borrow  Professor 
Young's  vivid  description,  "  stream  out  on  all  sides  radiant 
filaments,  beams,  and  sheets  of  pearly  light,  which  reach 
to  a  distance  sometimes  of  several  degrees  from  the  solar 
surface,  forming  an  irregular  stellate  halo,  with  the  black 
globe  of  the  moon  in  its  apparent  centre.  The  portion 
nearest  the  sun  is  of  dazzling  brightness,  but  still  less  bril- 
liant than  the  prominences  which  blaze  through  it  like 
carbuncles.  Generally  this  inner  corona  has  a  pretty  uni- 
form height,  forming  a  ring  three  or  four  minutes  of  arc 
in  width,  separated  by  a  somewhat  definite  outline  from 
the  outer  corona,  which  reaches  to  a  much  greater  dis- 
tance, and  is  far  more  irregular  in  form.  Usually  there  are 
several  rifts,  as  they  have  been  called,  like  narrow  beams  of 
darkness,  extending  from  the  very  edge  of  the  sun  to  the 
outer  night,  and  much  resembling  the  cloud-shadows  which 
radiate  from  the  sun  before  a  thunder-shower;  but  the 
edges  of  these  rifts  are  frequently  curved,  showing  them 


ASTRONOMY.  2O5 

to  be  something  else  than  real  shadows.  Sometimes  there 
are  narrow  bright  streamers,  as  long  as  the  rifts,  or  longer. 
These  are  often  inclined,  occasionally  are  even  nearly 
tangential  to  the  solar  surface,  and  frequently  are  curved. 
On  the  whole,  the  corona  is  usually  less  extensive  and 
brilliant  over  the  solar  poles,  and  there  is  a  recognizable 


Fig.  225. 

tendency  to  accumulations  above  the  middle  latitudes,  or 
spot-zones ;  so  that,  speaking  roughly,  the  corona  shows  a 
disposition  to  assume  the  form  of  a  quadrilateral  or  four- 
rayed  star,  though  in  almost  every  individual  case  this  form 
is  greatly  modified  by  abnormal  streamers  at  some  point  or 
other." 

203.  The    Corona   as   seen    at  Recent  Eclipses.  —  The 


2O6  ASTRONOMY. 

corona  can  be  seen  only  at  the  time  of  a  total  eclipse  of 
the  sun,  and  then  for  only  a  few  minutes.  Its  form  varies 
considerably  from  one  eclipse  to  another,  and  apparently 
also  during  the  same  eclipse.  At  least,  different  observers 
at  different  stations  depict  the  same  corona  under  very 
different  forms.  Fig.  225  represents  the  corona  of  1857  as 


Fig.  226. 

observed  by  Liais.  In  this  view  the  petal-like  forms,  which 
have  been  noticed  in  the  corona  at  other  times,  are  espe- 
cially prominent. 

Fig.  226  shows  the  corona  of  1860  as  it  was  observed 
by  Temple. 

Fig.  227  shows  the  corona  of  1867.  This  is  interesting 
as  being  a  corona  at  the  time  of  sun-spot  minimum. 


ASTRONOMY.  2O/ 

Fig.  228  represents  the  corona  of  1868.  This  is  a  larger 
and  more  irregular  corona  than  usual. 

The  corona  of  1869  is  shown  in  Fig.  229. 

Fig.  230  is  a  view  of  the  corona  of  1871  as  seen  by 
Capt.  Tupman. 

Fig.  231  shows  the  same  corona  as  seen  by  Fcenander. 


Fig.  227. 

Fig.  232  shows  the  same  corona  as  photographed  by 
Davis. 

Fig.  233  shows  the  corona  of  1878  made  up  from  several 
views  as  combined  by  Professor  Young. 

204.  The  Spectrum  of  the  Corona.  —  The  chief  line  in  the 
spectrum  of  the  corona  is  the  one  usually  designated  as  1474, 
and  now  known  as  the  coronal  line.  It  is  seen  as  a  dark  line 


208 


ASTRONOMY. 


on  the  disk  of  the  sun ;  and  a  spectroscope  of  great  dispersive 
power  shows  this  dark  line  to  be  closely  double,  the  lower 
component  being  one  of  the  iron  lines,  and  the  upper  the 
coronal  line.  This  dark  line  is  shown  at  xt  Fig.  234. 

Besides  this  bright  line,  the  hydrogen  lines  appear  faintly 
in  the   spectrum   of   the   corona.      The    1474  line    has   been 


Fig.  228. 

sometimes  traced  with  the  spectroscope  to  an  elevation  of 
nearly  twenty  minutes  above  the  moon's  limb,  and  the  hydro- 
gen lines  nearly  as  far;  and  the  lines  were  just  as  strong 
in  the  middle  of  a  dark  rift  as  anywhere  else. 

The  substance  which  produces  the  1474  line  is  unknown 
as  yet.  It  seems  to  be  something  with  a  vapor-density  far 
below  that  of  hydrogen,  which  is  the  lightest  substance  of 
which  we  have  any  knowledge.  I  can  hardly  be  an  "  allo- 


ASTRONOMY.  2OQ 

tropic  "  form  of  any  terrestrial  element,  as  some  scientists  have 
suggested ;  for  in  the  most  violent  disturbances  in  prominences 
and  near  sun-spots,  when  the  lines  of  hydrogen,  magnesium, 
and  other  metals,  are  contorted  and  shattered  by  the  rush  of 
the  contending  elements,  this  line  alone  remains  fine,  sharp, 
and  straight,  a  little  brightened,  but  not  otherwise  affected. 


Fig.    22Q. 

For  the  present  it  remains,  like  a  few  other  lines  in  the  spec- 
trum, an  unexplained  mystery. 

Besides  bright  lines,  the  corona  shows  also  a  faint  continu- 
ous spectrum,  in  which  have  been  observed  a  few  of  the  more 
prominent  dark  lines  of  the  solar  spectrum. 

This  shows,  that,  while  the  corona  may  be  in  the  main 
composed  of  glowing  gas  (as  indicated  by  the  bright  lines 
of  its  spectrum),  it  also  contains  considerable  matter  in  such 


2IO  ASTRONOMY. 

a  state  as  to  reflect  the  sunlight,  probably  in  the  form  of  dust 
or  fog. 

V.     ECLIPSES. 

205.  The    Shadows    of  the   Earth    and  Moon.  —  The 
shadows  cast  by  the  earth  and  moon  are  shown  in  Fig.  235. 


Fig.  230. 

Each  shadow  is  seen  to  be  made  up  of  a  dark  portion 
called  the  umbra,  and  of  a  lighter  portion  called  the 
penumbra.  The  light  of  the  sun  is  completely  excluded 
from  the  umbra,  but  only  partially  from  the  penumbra. 
The  umbra  is  in  the  form  of  a  cone,  with  its  apex  away 
from  the  sun  \  though  in  the  case  of  the  earth's  shadow 
it  tapers  very  slowly.  The  penumbra  surrounds  the  umbra. 


ASTRONOMY.  211 

and  increases  in  size  as  we  recede  from  the  sun.  The  axis 
of  the  earth's  shadow  lies  in  the  plane  of  the  ecliptic,  which 
in  the  figure  is  the  surface  of  the  page.  As  the  moon's 
orbit  is  inclined  five  degrees  to  the  plane  of  the  ecliptic, 
the  axis  of  the  moon's  shadow  will  sometimes  lie  above, 


Fig.  231. 

and  sometimes  below,  the  ecliptic.     It  will  lie  on  the  ecliptic 
only  when  the  moon  is  at  one  of  her  nodes. 

206.  When  there  will  be  an  Eclipse  of  the  Moon.  — 
The  moon  is  eclipsed  whenever  it  passes  into  the  umbra 
of  the  earth's  shadow.  It  will  be  seen  from  the  figure  that 
the  moon  can  pass  into  the  shadow  of  the  earth  only  when 
she  is  in  opposition,  or  at  full.  Owing  to  the  inclina- 
tion of  the  moon's  orbit  to  the  ecliptic,  the  moon  will  pass 


212 


ASTRONOMY. 


either  above  or  below  the  earth's  shadow  when  she  is  at 
full,  unless  she  happens  to  be  near  her  node  at  this  time  : 
hence  there  is  not  an  eclipse  of  the  moon  every  month. 

When  the  moon  simply  passes  into  the  penumbra  of  the 
earth's  shadow,  the  light  of  the  moon  is  somewhat  dimmed, 


Fig.  232. 

but  not  sufficiently  to  attract  attention,  or  to  be  denomi- 
nated an  eclipse. 

207.  The  Lunar  Ecliptic  Limits.  —  In  Fig.  236  the  line 
AB  represents  the  plane  of  the  ecliptic,  and  the  line  CD  the 
moon's  orbit.  The  large  black  circles  on  the  line  A  ^repre- 
sent sections  of  the  umbra  of  the  earth's  shadow,  and  the 
smaller  circles  on  CD  represent  the  moon  at  full.  It  will  be 
seen,  that,  if  the  moon  is  full  at  E,  she  will  just  graze  the 


ASTRONOMY.  213 

umbra  of  the  earth's  shadow.  In  this  case  she  will  suffer  no 
eclipse.  Were  the  moon  full  at  any  point  nearer  her  node,  as 
at  Fj  she  would  pass  into  the  umbra  of  the  earth's  shadow,  and 
would  be  partially  eclipsed.  Were  the  moon  full  at  G,  she 
would  pass  through  the  centre  of  the  earth's  shadow,  and  be 
totally  eclipsed. 

It  will  be  seen  from  the  figure  that  full   moon  must  occur 


Fig.  233. 

when  the  moon  is  within  a  certain  distance  from  her  node,  in 
order  that  there  may  be  a  lunar  eclipse ;  and  this  space  is 
called  the  lunar  ecliptic  limits. 

The  farther  the  earth  is  from  the  sun,  the  less  rapidly  does 
its  shadow  taper,  and  therefore  the  greater  its  diameter  at  the 
distance  of  the  moon ;  and,  the  nearer  the  moon  to  the  earth, 
the  greater  the  diameter  of  the  earth's  shadow  at  the  distance 
of  the  moon.  Of  course,  the  greater  the  diameter  of  the 


214 


ASTRONOMY. 


earth's  shadow,  the  greater  the  ecliptic  limits :  hence  the  lunar 
ecliptic  limits  vary  somewhat  from  time  to  time,  according  to 
the  distance  from  the  earth  to  the  sun  and  from  the  earth 
to  the  moon.  The  limits  within  which  an  eclipse  is  inevitable 
under  all  circumstances  are  called  the  minor  ecliptic  limits  ; 
and  those  within  which  an  eclipse  is  possible  under  some  cir- 
cumstances, the  major  ecliptic  limits. 

208.  Lunar  Eclipses.  —  Fig.  237  shows  the  path  of  the 
moon  through  the  earth's  shadow  in  the  case  of  a  partial 
eclipse.  The  magnitude  of  such  an  eclipse  depends  upon 
the  nearness  of  the  moon  to  her  nodes.  The  magnitude  of 
an  eclipse  is  usually  denoted  in  digits,  a  digit  being  one- 
twelfth  of  the  diameter  of  the  moon. 

Fig.  238  shows  the  path  of  the  moon  through  the  earth's 

shadow  in  the 
case  of  a  total 
eclipse.  It  will 
be  seen  from 
the  figure  that 
it  is  not  neces- 
sary for  the  moon 
to  pass  through 
the  centre  of  the 
earth's  shadow  in  order  to  have  a  total  eclipse.  When 
the  moon  passes  through  the  centre  of  the  earth's  shadow, 
the  eclipse  is  both  total  and  central. 

At  the  time  of  a  total  eclipse,  the  moon  is  not  entirely 
invisible,  but  shines  with  a  faint  copper-colored  light.  This 
light  is  refracted  into  the  shadow  by  the  earth's  atmosphere, 
and  its  amount  varies  with  the  quantity  of  clouds  and  vapor 
in  that  portion  of  the  atmosphere  which  the  sunlight  must 
graze  in  order  to  reach  the  moon. 

The  duration  of  an  eclipse  varies  between  very  wide 
limits,  being,  of  course,  greatest  when  the  eclipse  is  central. 
A  total  eclipse  of  the  moon  may  last  nearly  two  hours,  or, 


ASTRONOMY. 


215 


including  the  partial  portions  of  the  eclipse,  three  or  four 
hours. 


JO 


Fig.  235. 


Every  eclipse   of  the  moon,  whether  total  or  partial,  is 


216 


ASTRONOMY. 


visible  at  the  same  time  to  the  whole  hemisphere  of  the 
earth  which  is  turned  towards  the  moon ;  and  the  eclipse 
will  have  exactly  the  same  magnitude  at  every  point  of 
observation. 


Fig.  236. 

209.  When  there  will  be  an  Eclipse  of  the  Sun. — 
There  will  be  an  eclipse  of  the  sun  whenever  any  portion 
of  the  moon's  shadow  is  thrown  on  the  earth.  It  will  be 
seen  from  Fig.  235  that  this  can  occur  only  when  the  moon 


Fig.  237. 

is  in  conjunction,  or  at  new.  It  does  not  occur  every 
month,  because,  owing  to  the  inclination  of  the  moon's  orbit 
to  the  ecliptic,  the  moon's  shadow  is  usually  thrown  either 
above  or  below  the  earth  at  the  time  of  new  moon.  There 


ASTRONOMY.  217 

can  be  an  eclipse  of  the  sun  only  when  new  moon  occurs 
at  or  near  one  of  the  nodes  of  her  orbit. 

210.  Solar  Ecliptic  Limits.  —  The  distances  from  the  moon's 
node  within  which  a  new  moon  would  throw  some  portion  of 
its  shadow  on  the  earth  so  as  to  produce  an  eclipse  of  the 
sun  are  called  the  solar  ecliptic  limits.  As  in  the  case  of 
the  moon,  there  are  major  and  minor  ecliptic  limits ;  the  former 
being  the  limits  within  which  an  eclipse  of  the  sun  is  possi- 
ble under  some  circumstances,  and  the  latter  those  under  which 
an  eclipse  is  inevitable  under  all  circumstances. 


Fig.  238. 

The  limits  within  which  a  solar  eclipse  may  occur  are 
greater  than  those  within  which  a  lunar  eclipse  may  occur. 
This  will  be  evident  from  an  examination  of  Fig.  235.  Were 
the  moon  in  that  figure  just  outside  of  the  lines  A  B  and  CD, 
it  will  be  seen  that  the  penumbra  of  her  shadow  would  just 
graze  the  earth :  hence  the  moon  must  be  somewhere  within 
the  space  bounded  by  these  lines  in  order  to  cause  an  eclipse 
of  the  sun.  Now,  these  lines  mark  the  prolongation  to  the 
sun  of  the  cone  of  the  umbra  of  the  earth's  shadow:  hence, 
in  order  to  produce  an  eclipse  of  the  sun,  new  moon  must 
occur  somewhere  within  this  prolongation  of  the  umbra  of  the 
earth's  shadow.  Now,  it  is  evident  that  the  diameter  of  this 


218 


ASTRONOMY. 


cone  is  greater  on  the  side  of  the  earth  toward  the  sun  than 
on  the  opposite  side :  hence  the  solar  ecliptic  limits  are  greater 
than  the  lunar  ecliptic  limits. 

211.  Solar  Eclipses.  —  An  observer  in  the  umbra  of  the 
moon's  shadow  would  see  a  total  eclipse  of  the  sun,  while 


Fig.  239. 

one  in  the  penumbra  would  see  only  a  partial  eclipse.  The 
magnitude  of  this  partial  eclipse  would  depend  upon  the 
distance  of  the  observer  from  the  umbra  of  the  moon's 
shadow. 

The  umbra  of   the  moon's  shadow  is  just   about   long 


Fig.  240. 

enough  to  reach  the  earth.  Sometimes  the  point  of  this 
shadow  falls  short  of  the  earth's  surface,  as  shown  in 
Fig.  239,  and  sometimes  it  falls  upon  the  earth,  as  shown 
in  Fig.  240,  according  to  the  varying  distance  of  the  sun 
and  moon  from  the  earth.  The  diameter  of  the  umbra  at 
the  surface  of  the  earth  is  seldom  more  than  a  hundred 


ASTRONOMY.  2IQ 

miles :  hence  the  belt  of  a  total  eclipse  is,  on  the  average, 
not  more  than  a  hundred  miles  wide ;  and  a  total  eclipse 
seldom  lasts  more  than  five  or  six  minutes,  and  sometimes 
only  a  few  seconds.  Owing,  however,  to  the  rotation  of 
the  earth,  the  umbra  of  the  moon's  shadow  may  pass  over 
a  long  reach  of  the  earth's  surface.  Fig.  241  shows  the 


Fig.  241. 

track  of  the  umbra  of  the  moon's  shadow  over  the  earth 
in  the  total  eclipse  of  1860. 

Fig.  242  shows  the  track  of  the  total  eclipse  of  1871 
across  India  and  the  adjacent  seas. 

In  a  partial  eclipse  of  the  sun,  more  or  less  of  one  side 
of  the  sun's  disk  is  usually  concealed,  as  shown  in  Fig.  243. 
Occasionally,  however,  the  centre  of  the  sun's  disk  is  cov- 
ered, leaving  a  bright  ring  around  the  margin,  as  shown  in 
Fig.  244.  Such  an  eclipse  is  called  an  annular  eclipse. 


220 


ASTRONOMY. 


An   eclipse   can   be   annular  only  when   the   cone  of  the 
moon's  shadow  is  too  short  to  reach  the  earth,  and  then 


Fig.  242. 

only  to   observers  who   are  in  the   central   portion  of  the 
penumbra. 

212.    Comparative    Frequency    of    Solar    and    Lunar 
Eclipses.  —  There  are  more  eclipses  of  the  sun  in  the  year 


ASTRONOMY.  221 

than  of  the  moon ;  and  yet,  at  any  one  place,  eclipses  of 
the  moon  are  more  frequent  than  those  of  the  sun. 

There  are  more  lunar  than  solar  eclipses,  because,  as  we 
have  seen,  the  limits  within  which  a  solar  eclipse  may  occur 
are  greater  than  those  within  which  a  lunar  eclipse  may  occur. 
There  are  more  eclipses  of  the  moon  visible  at  any  one  place 
than  of  the  sun ;  because,  as  we  have  seen,  an  eclipse  of  the 


Fig.  244. 


Fig.  243. 

moon,  whenever  it  does  occur,  is  visible  to  a  whole  hemisphere 
at  a  time,  while  an  eclipse  of  the  sun  is  visible  to  only  a  por- 
tion of  a  hemisphere,  and  a  total  eclipse  to  only  a  very  small 
portion  of  a  hemisphere.  A  total  eclipse  of  the  sun  is,  there- 
fore, a  very  rare  occurrence  at  any  one  place. 

The  greatest  number  of  eclipses  that  can  occur  in  a  year  is 
seven,  and  the  least  number,  two.  In  the  former  case,  five 
may  be  of  the  sun  and  two  of  the  moon,  or  four  of  the  sun 
and  three  of  the  moon.  In  the  latter  case,  both  must  be  of  the 
sun. 

VI.     THE   THREE   GROUPS   OF   PLANETS. 

I.      GENERAL     CHARACTERISTICS     OF    THE 
GROUPS. 

213.  The  Inner  Group.  —  The  inner  group  of  planets 
is  composed  of  Mercury,  Venus,  the  Earth,  and  Mars; 
that  is,  of  all  the  planets  which  lie  between  the  asteroids 


222 


ASTRONOMY. 


and  the  sun.  The  planets  of  this  group  are  comparatively 
small  and  dense.  So  far  as  known,  they  rotate  on  their 
axes  in  about  twenty-four  hours,  and  they  are  either  entirely 
without  moons,  or  are  attended  by  comparatively  few. 

The  comparative  sizes  and  eccentricities  of  the  orbits  of 


Fig.  245. 

this  group  are  shown  in  Fig.  245.    The  dots  round  the  orbits 
show  the  position  of  the  planets  at  intervals  of  ten  days. 

214.  The  Outer  Group. — The  outer  group  of  planets 
is  composed  of  Jupiter,  Saturn,  Uranus,  and  Neptune. 
These  planets  are  all  very  large  and  of  slight  density.  So 
far  as  known,  they  rotate  on  their  axes  in  about  ten  hours, 


ASTRONOMY. 


223 


and  are  accompanied  with  complicated  systems  of  moons. 
Fig.  246,  which  represents  the  comparative  sizes  of  the 
planets,  shows  at  a  glance  the  immense  difference  between 
those  of  the  inner  and  outer  group.  Fig.  247  shows  the 
comparative  sizes  and  eccentricities  of  the  orbits  of  the 
outer  planets.  The  dots  round  the  orbits  show  the  position 
of  the  planets  at  intervals 
of  a  thousand  days. 

215.  The  Asteroids. — 
Between  the  inner  and 
outer  groups  of  planets 
there  is  a  great  number 
of  very  small  planets 
known  as  the  minor  plan- 
ets, or  asteroids.  Over 
two  hundred  planets  be- 
longing to  this  group 
have  already  been  dis- 
covered. Their  orbits  are 
shown  by  the  dotted  lines 
in  Fig.  247.  The  sizes 
of  the  four  largest  of 
these  planets,  compared 
with  the  earth,  are  shown 
in  Fig.  248. 

The    asteroids    of    this 


Fig.  246. 


group  are  distinguished  from  the  other  planets,  not  only  by 
their  small  size,  but  by  the  great  eccentricities  and  inclina- 
tions of  their  orbits.  If  we  except  Mercury,  none  of  the 
larger  planets  has  an  eccentricity  amounting  to  one-tenth 
the  diameter  of  its  orbit  (43),  nor  is  any  orbit  inclined  more 
than  two  or  three  degrees  to  the  ecliptic ;  but  the  inclina- 
tions of  many  of  the  minor  planets  exceed  ten  degrees,  and 
the  eccentricities  frequently  amount  to  an  eighth  of  the 
orbital  diameter.  The  orbit  of  Pallas  is  inclined  thirty-four 


224 


ASTRONOMY, 


degrees  to  the  ecliptic,  while  there  are  some  planets  of 
this  group  whose  orbits  nearly  coincide  with  the  plane  of 
the  ecliptic. 

Fig.  249  shows  one  of  the  most  and  one  of  the  least 


eccentric  of  the  orbits  of  this  group  as  compared  with  that 
of  the  earth. 

The  intricate  complexity  of  the  orbits  of  the  asteroids  is 
shown  in  Fig.  250. 


ASTRONOMY. 


225 


II.     THE    INNER   GROUP   OF   PLANETS. 
MERCURY. 

216.  The  Orbit  of  Mercury. — The  orbit  of  Mercury  is 
more  eccentric  than  that  of  any  of  the  larger  planets,  and 
it  has  also  a  greater 
inclination  to  the  eclip- 
tic. Its  eccentricity  (43) 
is  a  little  over  a  fifth, 
and  its  inclination  to 
the  ecliptic  somewhat 
over  seven  degrees.  The 
mean  distance  of  Mer- 
cury from  the  sun  is 
about  thirty-five  million  miles ;  but,  owing  to  the  great 
eccentricity  of  its  orbit,  its  distance  from  the  sun  varies 

from  about  forty- 
three  million  miles 
at  aphelion  to 
about  twenty-eight 
million  at  perihe- 
lion. 

217.  Distance  of 
Mercury  from  the 
Earth.  —  It  is  evi- 
dent, from  Fig. 
251,  that  an  infe- 
rior planet,  like 
Mercury,  is  the 


FreUt 

Fig.  249. 


whole  diameter  of 
its  orbit  nearer  the 
earth  at  inferior  conjunction  than  at  superior  conjunction : 
hence  Mercury's  distance  from  the  earth  varies  considerably. 
Owing  to  the  great  eccentricity  of  its  orbit,  its  distance 


226  ASTRONOMY. 

from  the  earth  at  inferior  conjunction  also  varies  considera- 
bly. Mercury  is  nearest  to  the  earth  when  its  inferior 
conjunction  occurs  at  its  own  aphelion  and  at  the  earth's 
perihelion. 

218.  Apparent  Size  of  Mercury.  —  Since  Mercury's  dis- 
tance from  the  earth  is  variable,  the  apparent  size  of  the 


Fig.  250. 

planet  is  also  variable.  Fig.  252  shows  its  apparent  size  at 
its  extreme  and  mean  distances  from  the  earth.  Its  appar- 
ent diameter  varies  from  five  seconds  to  twelve  seconds. 

219.  Volume  and  Density  of  Mercury.  —  The  real 
diameter  of  Mercury  is  about  three  thousand  miles.  Its 
size,  compared  with  that  of  the  earth,  is  shown  in  Fig. 
253.  The  earth  is  about  sixteen  times  as  large  as  Mer- 


ASTRONOMY. 


227 


cury ;  but  Mercury  is  about  one-fifth  more  dense  than  the 
earth. 

220.  Greatest  Elongation  of  Mercury.  —  Mercury,  being 
an  inferior  planet  (or  one  within  the  orbit  of  the  earth), 
appears    to     oscillate 

to  and  fro  across 
the  sun.  Its  great- 
est apparent  distance 
from  the  sun,  or  its 
greatest  elongation, 
varies  considerably. 
The  farther  Mercury 
is  from  the  sun,  and 
the  nearer  the  earth 
is  to  Mercury,  the 
greater  is  its  angular 
distance  from  the  sun 
at  the  time  of  its 
greatest  elongation.  Under  the  most  favorable  circum- 
stances, the  greatest  elongation  amounts  to  about  twenty- 
eight  degrees,  and  under  the  least  favorable  to  only  sixteen 
or  seventeen  degrees. 

221.  Sidereal  and  Synodical  Periods  of  Mercury.  —  Mer- 

cury accomplishes 
a  complete  revolu- 
tion around  the  sun 
in  about  eighty- 
eight  days  ;  but  it 
takes  it  a  hundred 
Fis-  252.  and  sixteen  days  to 

pass  from  its  greatest  elongation  east  to  the  same  elonga- 
tion again.  The  orbital  motion  of  this  planet  is  at  the 
rate  of  nearly  thirty  miles  a  second. 

In   Fig.  251,  P'"  represents  elongation  east  of  the  sun, 
and  P'  elongation  west.     It  will  be  seen  that  it  is  much 


228 


ASTRONOMY. 


farther  from  P'  around  to  P'"  than  from  P'"  on  to  P'. 
Mercury  is  only  about  forty-eight  days  in  passing  from 
greatest  elongation  east  to  greatest  elongation  west,  while 
it  is  about  sixty-eight  days  in  passing  back  again. 

222.    Visibility  of  Mercury.  —  Mercury  is    too    close    to 


Fig.  253. 

the  sun  for  favorable  observation.  It  is  never  seen  long 
after  sunset,  or  long  before  sunrise,  and  never  far  from  the 
horizon.  When  visible  at  all,  it  must  be  sought  for  low 
down  in  the  west  shortly  after  sunset,  or  low  in  the  east 

shortly  before  sunrise,  accord- 
ing as  the  planet  is  at  its  east 
or  west  elongation.  It  is  often 
visible  to  the  naked  eye  in  our 
latitude;  but  the  illumination 
of  the  twilight  sky,  and  the 
excess  of  vapor  in  our  atmos- 
phere near  the  horizon,  com- 
bine to  make  the  telescopic 
study  of  the  planet  difficult 
and  unsatisfactory. 
223.  The  Atmosphere  and  Surface  of  Mercury.  —  Mer- 
cury seems  to  be  surrounded  by  a  dense  atmosphere.  One 
proof  of  the  existence  of  such  an  atmosphere  is  furnished 
at  the  time  of  the  planet's  transit  across  the  disk  of  the 
sun,  which  occasionally  happens.  The  planet  is  then  seen 


Fig.  254. 


ASTRONOMY. 


229 


surrounded  by  a  border,  as  shown  in  Fig.  254.  A  bright 
spot  has  also  been  observed  on  the  dark  disk  of  the  planet 
during  a  transit,  as  shown  in  Fig.  255.  The  border  around 
the  planet  seems  to 
be  due  to  the  action 
of  the  planet's  atmos- 
phere; but  it  is  dif- 
ficult to  account  for 
the  bright  spot. 

Schroter,  a  cele- 
brated German  as- 
tronomer, at  about  the 
beginning  of  the  pres- 
ent century,  thought 
that  he  detected  spots 
and  shadings  on  the 
disk  of  the  planet, 
which  indicated  both  Fig-  255- 

the  presence  of  an  atmosphere  and  of  elevations.-  The 
shading  along  the  terminator,  which  seemed  to  indicate 
the  presence  of  a  twilight,  and  therefore  of  an  atmos- 
phere, are  shown  in  Fig.  256.  It  also  shows  the  blunted 


Fig.  256. 

aspect  of  one  of  the  cusps,  which  Schroter  noticed  at  times, 
and  which  he  attributed  to  the  shadow  of  a  mountain, 
estimated  to  be  ten  or  twelve  miles  high.  Fig.  257  shows 


23O  ASTRONOMY. 

this  mountain  near  the  upper  cusp,  as  Schroter  believed  he 
saw  it  in  the  year  1 800.  By  watching  certain  marks  upon 
the  disk  of  Mercury,  Schroter  came  to  the  conclusion  that 
the  planet  rotates  on  its  axis  in  about  twenty -four  hours. 
Modern  observers,  with  more  powerful  telescopes,  have 
failed  to  verify  Schroter's  observations  as  to  the  indications 
of  an  atmosphere  and  of  elevations.  Nothing  is  known 
with  certainty  about  the  rotation  of  the  planet. 

The  border  around  Mercury,  and  the  bright  spot  on  its 
disk  at  the  time  of  the  transit  of  the  planet  across  the  sun, 
have  been  seen  since  Schroter's  time,  and  the  existence  of 
these  phenomena  is  now  well  established ;  but  astronomers 
are  far  from  being  agreed  as  to 
their  cause. 

224.  Infra- Mercurial  Planets. 
—  It  has  for  some  time  been 
thought  probable  that  there  is  a 
group  of  small  planets  between 
Mercury  and  the  sun ;  and  at  vari- 
ous times  the  discovery  of  such 
bodies  has  been  announced.  In 
1859  a  French  observer  believed 
Fig.  257.  that  he  had  detected  an  intra- 

Mercurial  planet,  to  which  the  name  of  Vulcan  was  given, 
and  for  which  careful  search  has  since  been  made,  but  with- 
out success.  During  the  total  eclipse  of  1878  Professor 
Watson  observed  two  objects  near  the  sun,  which  he  thought 
to  be  planets ;  but  this  is  still  matter  of  controversy. 

VENUS. 

225.  The  Orbit  of  Venus. — The  orbit  of  Venus  has 
but  slight  eccentricity,  differing  less  from  a  circle  than  that 
of  any  other  large  planet.  It  is  inclined  to  the  ecliptic  some- 
what more  than  three  degrees.  The  mean  distance  of  the 
planet  from  the  sun  is  about  sixty-seven  million  miles. 


ASTRONOMY.  231 

226.  Distance   of   Vemis  from    the   Earth. — The    dis- 
tance of  Venus  from  the  earth  varies  within  much  wider 
limits  than  that  of  Mercury.     When  Venus  is  at  inferior 
conjunction,    her   distance   from   the   earth    is    ninety-two 
million  miles  minus  sixty-seven  million   miles,  or  twenty- 
five  million  miles ;  and  when  at  superior  conjunction  it  is 
ninety-two  million  miles  plus  sixty-seven  million  miles,  or 
a  hundred  and  fifty-nine  million  miles.     Venus  is  consid- 
erably more  than  six  times  as  far  off  at  superior  conjunc- 
tion as  at  inferior  conjunction. 

227.  Apparent    Size    of    Venus.  —  Owing   to   the   great 


Fig.  258. 

variation  in  the  distance  of  Venus  from  the  earth,  her 
apparent  diameter  varies  from  about  ten  seconds  to  about 
sixty-six  seconds.  Fig.  258  shows  the  apparent  size  of 
Venus  at  her  extreme  and  mean  distances  from  the  earth. 

228.  Volume  and  Density  of  Venus. — The  real  size  of 
Venus  is  about  the  same  as  that  of  the  earth,  her  diameter 
being  only  about  three  hundred  miles  less.     The  compara- 
tive sizes  of  the  two  planets  are  shown  in  Fig.  259.     The 
density  of  Venus  is  a  little  less  than  that  of  the  earth. 

229.  The   Greatest  Elongation    of  Venus. — Venus,  like 
Mercury,  appears  to  oscillate   to   and  fro   across  the   sun. 
The   angular   value    of    the   greatest   elongation   of   Venus 
varies  but  slightly,  its  greatest  value  being  about  forty-seven 
degrees. 


232  ASTRONOMY. 

230.  Sidereal  and  Synodic al  Periods  of  Venus.  —  The 
sidereal  period  of  Venus,  or  that  of  a  complete  revolution 
around  the  sun,  is  about  two  hundred  and  twenty-five  days  ; 
her  orbital  motion  being  at  the  rate  of  nearly  twenty-two 
miles  a  second.  Her  synodical  period,  or  the  time  it  takes 
her  to  pass  around  from  her  greatest  eastern  elongation 
to  the  same  elongation  again,  is  about  five  hundred  and 
eighty-four  days,  or  eighteen  months.  Venus  is  a  hun- 
dred and  forty-six  days,  or  nearly  five  months,  in  passing 
from  her  greatest  elongation  east  through  inferior  conjunc- 
tion to  her  greatest  elongation  west. 


Fig.  259. 

231.  Venus  as  a  Morning  and  an  Evening  Star.  —  For 
a  period  of  about  nine  months,  while  Venus  is  passing  from 
superior  conjunction  to  her  greatest  eastern  elongation,  she 
will  be  east  of  the  sun,  and  will    therefore    set  after   the 
sun.     During  this  period  she  is  the  evening  star,  the  Hespe- 
rus of  the  ancients.     While  passing  from  inferior  conjunc- 
tion to  superior  conjunction,  Venus  is  west  of  the  sun,  and 
therefore  rises  before  the  sun.     During  this  period  of  nine 
months  she  is  the  morning  star,  the  Phosphorus,  or  Lucifer, 
of  the  ancients. 

232.  Brilliancy  of  Venus.  —  Next  to  the  sun  and  moon, 
Venus  is  at  times  the  most  brilliant  object  in  the  heavens, 
being  bright  enough  to  be  seen  in  daylight,  and  to  cast 


ASTRONOMY.  233 

a  distinct  shadow  at  night.  Her  brightness,  however,  varies 
considerably,  owing  to  her  phases  and  to  her  varying  dis- 
tance from  the  earth.  She  does  not  appear  brightest  when 
at  full,  for  she  is  then  farthest  from  the  earth,  at  superior 
conjunction  ;  nor  does  she  appear  brightest  when  nearest  the 
earth,  at  inferior  conjunction,  for  her  phase  is  then  a  thin 
crescent  (see  Fig.  258).  She  is  most  conspicuous  while 
passing  from  her  greatest  eastern  elongation  to  her  great- 
est western  elongation.  After  she  has  passed  her  eastern 
elongation,  she  becomes  brighter  and  brighter  till  she  is 
within  about  forty  degrees  of  the  sun.  Her  phase  at 
this  point  in  her  orbit  is 
shown  in  Fig.  260.  Her 
brilliancy  then  begins  to 
wane,  until  she  comes  too 
near  the  sun  to  be  visi- 
ble. When  she  re-appears 
on  the  west  of  the  sun, 
she  again  becomes  more 
brilliant ;  and  her  brilliancy 
increases  till  she  is  about 
forty  degrees  from  the  sun, 
when  she  is  again  at  her 
brightest.  Venus  passes 

from  her  greatest  brilliancy  as  an  evening  star  to  her  great- 
est brilliancy  as  a  morning  star  in  about  seventy-three  days. 
She  has  the  same  phase,  and  is  at  the  same  distance  from 
the  earth,  in  both  cases  of  maximum  brilliancy.  Of  course, 
the  brilliancy  of  Venus  when  at  the  maximum  varies  some- 
what from  time  to  time,  owing  to  the  eccentricities  of  the 
orbits  of  the  earth  and  of  Venus,  which  cause  her  distance 
from  the  earth,  at  her  phase  of  greatest  brilliancy,  to  vary. 
She  is  most  brilliant  when  the  phase  of  her  greatest  bril- 
liancy occurs  when  she  is  at  her  aphelion  and  the  earth  at 
its  perihelion. 


234     ,  ASTRONOMY. 

233.  The  Atmosphere  and  Surface  of  Venus.  —  Schroter 
believed  that  he  saw  shadings  and  markings  on  Venus  simi- 
lar to  those  on  Mercury,  indicating  the  presence  of  an 
atmosphere  and  of  elevations  on  the  surface  of  the  planet. 
Fig.  261  represents  the  surface  of  Venus  as  it  appeared 


Fig.  261. 

to  this  astronomer.  By  watching  certain  markings  on  the 
disk  of  Venus,  Schroter  came  to  the  conclusion  that  Venus 
rotates  on  her  axis  in  about  twenty-four  hours. 

It  is  now  generally  conceded  that  Venus  has  a  dense 
atmosphere ;  but  Schroter's  obser- 
vations of  the  spots  on  her  disk 
have  not  been  verified  by  modern 
astronomers,  and  we  really  know 
nothing  certainly  of  her  rotation. 

234.  Transits  of  Venus.  —  When 
Venus  happens  to  be  near  one  of 
the  nodes  of  her  orbit  when  she  is 
in  inferior  conjunction,  she  makes 
a  transit  across  the  sun's  disk. 

These  transits  occur  in  pairs,  separated  by  an  interval  of 
over  a  hundred  years.  The  two  transits  of  each  pair  are 
separated  by  an  interval  of  eight  years,  the  dates  of  the  most 
recent  being  1874  and  1882.  Venus,  like  Mercury,  appears 
surrounded  with  a  border  on  passing  across  the  sun's  disk, 
as  shown  in  Fig.  262. 


ASTRONOMY. 


235 


MARS. 

235.  The  Orbit  of  Mars.  —  The  orbit  of  Mars  is  more 
eccentric  than   that   of  any  of  the   larger  planets,  except 
Mercury;   its  eccentricity  being  about  one-eleventh.     The 
inclination  of  the  orbit  to  the  ecliptic  is  somewhat  under 
two  degrees.     The  mean  distance  of  Mars  from  the  sun 
is  about  a  hundred  and  forty  million  miles ;  but,  owing  to 
the  eccentricity  of  his  orbit,  the  distance  varies  from  a  hun- 
dred and  fifty-three  million  miles  to  a  hundred  and  twenty- 
seven  million  miles. 

236.  Distance  of 
Mars     from      the 
Earth.  —  It  will  be 
seen,  from  Fig.  263, 
that      a      superior 
planet  (or  one  out- 
side    the    orbit   of 
the      earth),      like 
Mars,  is  nearer  the 
earth,  by  the  whole 
diameter      of     the 
earth's  orbit,  when 

in  opposition    than  Fig.  263. 

when  in  conjunction.  The  mean  distance  of  Mars  from 
the  earth,  at  the  time  of  opposition,  is  a  hundred  and  forty 
million  miles  minus  ninety-two  million  miles,  or  forty-eight 
million  miles.  Owing  to  the  eccentricity  of  the  orbit  of 
the  earth  and  of  Mars,  the  distance  of  this  planet  when 
in  opposition  varies  considerably.  When  the  earth  is  in 
aphelion,  and  Mars  in  perihelion,  at  the  time  of  opposition, 
the  distance  of  the  planet  from  the  earth  is  only  about 
thirty-three  million  miles.  On  the  other  hand,  when  the 
earth  is  in  perihelion,  and  Mars  in  aphelion,  at  the  time  of 
opposition,  the  distance  of  the  planet  is  over  sixty-two 
million  miles. 


236  ASTRONOMY. 

The  mean  distance  of  Mars  from  the  earth  when  in 
conjunction  is  a  hundred  and  forty  million  miles  plus 
ninety-two  million  miles,  or  two  hundred  and  thirty-two 
million  miles.  It  will  therefore  be  seen  that  Mars  is  nearly 
five  times  as  far  off  at  conjunction  as  at  opposition. 


Fig.  264. 

237.  The  Apparent  Size  of  Mars.  —  Owing  to  the  vary- 
ing distance  of  Mars  from  the  earth,  the  apparent  size  of 
the  planet  varies  almost  as  much  as  that  of  Venus.  Fig. 
264  shows  the  apparent  size  of  Mars  at  its  extreme  and 
mean  distances  from  the  earth.  The  apparent  diameter 
varies  from  about  four  seconds  to  about  thirty  seconds. 


Fig.  265. 

238.  The  Volume  and  Density  of  Mars.  —  Among  the 
larger  planets  Mars  is  next  in  size  to  Mercury.  Its  real 
diameter  is  somewhat  more  than  four  thousand  miles,  and 
its  bulk  is  about  one-seventh  of  that  of  the  earth.  Its  size, 
compared  with  that  of  the  earth,  is  shown  in  Fig.  265. 


PLATE  W. 


ASTRONOMY.  23/ 

4 

The  density  of  Mars  is  only  about  three- fourths  of  that 
of  the  earth. 

239.  Sidereal  and  Sy nodical  Periods   of  Mars.  —  The 
sidereal  period  of  Mars,  or  the  time  in  which  he  makes  a 
complete  revolution  around  the  sun,  is  about  six  hundred 
and  eighty-seven  days,  or  nearly  twenty-three  months ;  but 
he  is  about  seven  hundred  and  eighty  days  in  passing  from 
opposition  to  opposition  again,  or  in  performing  a  synodical 
revolution.     Mars  moves  in  his  orbit  at  the  rate  of  about 
fifteen  miles  a  second. 

240.  Brilliancy   of  Mars.  —  When  near  his  opposition, 
Mars  is  easily  recognized  with  the  naked  eye  by  his  fiery-red 
light.     He  is  much  more  brilliant  at  some  oppositions  than 
at  others,  for  reasons  already  explained  (236),  but  always 
shines  brighter  than  an  ordinary  star  of  the  first  magnitude. 

241.  Telescopic   Appearance   of   Mars.  —  When   viewed 
with  a  good  telescope  (see  Plate  IV.),  Mars  is  seen  to  be 
covered  with  dusky,  dull-red  patches,  which  are  supposed 
to  be  continents,  like  those  of  our  own  globe.     Other  por- 
tions, of  a  greenish  hue,  are  believed  to  be  tracts  of  water. 
The  ruddy  color,  which  overpowers  the  green,  and  makes 
the  whole  planet  seem  red  to  the  naked  eye,  was  believed 
by  Sir  J.  Herschel  to   be   due  to  an  ochrey  tinge  in  the 
general  soil,  like  that  of  the  red  sandstone  districts  on  the 
earth.    In  a  telescope,  Mars  appears  less  red,  and  the  higher 
the  power  the  less  the  intensity  of  the  color.     The  disk, 
when  well  seen,  is  mapped  out  in  a  way  which  gives  at  once 
the  impression  of  land  and  water.     The  bright  part  is  red  in- 
clining to  orange,  sometimes  dotted  with  brown  and  greenish 
points.     The  darker  spaces,  which  vary  greatly  in  depth  of 
tone,  are  of  a  dull  gray-green,  having  the  aspect  of  a  fluid 
which  absorbs  the  solar  rays.     The  proportion  of  land  to 
water  on  the  earth  appears  to  be  reversed  on  Mars.     On  the 
earth  every  continent  is  an  island ;    on  Mars  all  seas  are 
lakes.     Long,  narrow  straits  are  more  common  than  on  the 


238 


ASTRONOMY. 


earth ;  and  wide  expanses  of  water,  like  our  Atlantic  Ocean, 
are  rare.     (See  Fig.  -266.) 


Fig.  266. 

Fig.  267  represents  a  chart  of  the  surface  of  Mars,  which 


Fig.  267. 

has  been   constructed  from  careful  telescopic  observation. 
The  outlines,  as  seen  in  the  telescope,  are,  however,,  much 


ASTRONOMY.  239 

less  distinct  than  they  are  represented  here ;  and  it  is  by 
no  means  certain  that  the  light  and  dark  portions  are  bodies 
of  land  and  water. 

In  the  vicinity  of  the  poles  brilliant  white  spots  may  be 
noticed,  which  are  considered  by  many  astronomers  to  be 
masses  of  snow.  This  conjecture  is  favored  by  the  fact 
that  they  appear  to  diminish  under  the  sun's  influence  at 
the  beginning  of  the  Martial  summer,  and  to  increase  again 
on  the  approach  of  winter. 

242.  Rotation  of  Mars.  —  On  watching  Mars  with  a 
telescope,  the  spots  on  the  disk  are  found  to  move  (as 
shown  in  Fig.  268)  in  a  manner  which  indicates  that  the 


Fig.  268. 

planet  rotates  in  about  twenty-four  hours  on  an  axis  in- 
clined about  twenty-eight  degrees  from  a  perpendicular 
to  the  plane  of  its  orbit.  The  inclination  of  the  axis  is 
shown  in  Fig.  269.  It  is  evident  from  the  figure  that  the 
variation  in  the  length  of  day  and  night,  and  the  change 
of  seasons,  are  about  the  same  on  Mars  as  on  the  earth. 
The  changes  will,  of  course,  be  somewhat  greater,  and  the 
seasons  will  be  about  twice  as  long. 

243.  The  Satellites  of  Mars.  —  In  1877  Professor  Hall 
of  the  Washington  Observatory  discovered  that  Mars  is 
accompanied  by  two  small  moons,  whose  orbits  are  shown 
in  Fig.  270.  The  inner  satellite  has  been  named  Phobos, 
and  the  outer  one  Deimos.  It  is  estimated  that  the  diame- 
ter of  the  outer  moon  is  from  five  to  ten  miles,  and  that 
of  the  inner  one  from  ten  to  forty  miles. 


24O  ASTRONOMY. 

Phobos  is  remarkable  for  its  nearness  to  the  planet  and 
the  rapidity  of  its  revolution,  which  is.  performed  in  seven 
hours  thirty-eight  minutes.  Its  distance  from  the  centre  of 


Fig.  269. 

the  planet  is  about  six  thousand  miles,  and  from  the  surface 
less  than  four  thousand.  Astronomers  on  Mars,  with  tele- 
scopes and  eyes  like  ours,  could  readily  find  out  whether 


Fig.  270. 

this  satellite  is  inhabited,  the  distance  being  less  than  one- 
sixtieth  of  that  of  our  moon. 

It  will  be  seen  that  Phobos  makes  about  three  revolutions 


ASTRONOMY.  24! 

to  one  rotation  of  the  planet.  It  will,  of  course,  rise  in  the 
west ;  though  the  sun,  the  stars,  and  the  other  satellite  rise 
in  the  east.  Deimos  makes  a  complete  revolution  in  about 
thirty  hours. 

III.     THE   ASTEROIDS. 

244.  Bode's  Law  of  Planetary  Distances.  —  There  is  a 
very  remarkable  law  connecting  the  distances  of  the  planets 
from  the  sun,  which  is  generally  known  by  the  name  of 
Bode's  Law.     Attention  was  drawn  to   it  in  1778  by  the 
astronomer  Bode,  but  he  was  not  really  its  author. 

To  express  this  law  we  write  the  following  series  of  num- 
bers :  — 

o,  3,  6,   12,  24,  48,  96; 

each  number,  with  the  exception  of  the  first,  being  double 
the  one  which  precedes  it.  If  we  add  4  to  each  of  these 
numbers,  the  series  becomes  — 

4,   7,   10,   1 6,  28,  52,   100 ; 

which  series  was  known  to  Kepler.  These  numbers,  with 
the  exception  of  28,  are  sensibly  proportional  to  the  dis- 
tances of  the  principal  planets  from  the  sun,  the  actual 
distances  being  as  follows  :  — 

Mercury.     Venus.     Earth.     Mars.     Jupiter.     Saturn. 

3*9         7'2       I0       T5'2  52*9      95'4 

245.  The  First  Discovery  of  the  Asteroids.  —  The  great 
gap  between  Mars  and  Jupiter  led  astronomers,  from  the 
time  of  Kepler,  to  suspect  the   existence  of  an  unknown 
planet  in  this  region ;  but  no  such  planet  was  discovered 
till  the  beginning  of  the  present  century.     Ceres  was  dis- 
covered Jan.  i,  1801,  Pallas  in   1802,  Juno  IK   1804,  and 
Vesta  in   1807.     Then  followed  a  long   interval  of  thirty- 
eight  years  before  Astrtza,  the  fifth  of  these  minor  planets, 
was  discovered  in  1845. 

246.  Olbers's  Hypothesis.  —  After  the  discovery  of  Pallas, 


242  ASTRONOMY. 

Olbers  suggested  his  celebrated  hypothesis,  that  the  two 
bodies  might  be  fragments  of  a  single  planet  which  had 
been  shattered  by  some  explosion.  If  such  were  the  case, 
the  orbits  of  all  the  fragments  would  at  first  intersect  each 
other  at  the  point  where  the  explosion  occurred.  He  there- 
fore thought  it  likely  that  other  fragments  would  be  found, 
especially  if  a  search  were  kept  up  near  the  intersection  of 
the  orbits  of  Ceres  and  Pallas. 

Professor  Newcomb  makes  the  following  observations  con- 
cerning this  hypothesis  :  — 

"  The  question  whether  these  bodies  could  ever  have  formed 
a  single  one  has  now  become  one  of  cosmogony  rather  than  of 
astronomy.  If  a  planet  were  shattered,  the  orbit  of  each  frag- 
ment would  at  first  pass  through  the  point  at  which  the  explo- 
sion occurred,  however  widely  they  might  be  separated  through 
the  rest  of  their  course ;  but,  owing  to  the  secular  changes 
produced  by  the  attractions  of  the  other  planets,  this  coinci- 
dence would  not  continue.  The  orbits  would  slowly  move 
away,  and  after  the  lapse  of  a  few  thousand  years  no  trace 
of  a  common  intersection  would  be  seen.  It  is  therefore 
curious  that  Olbers  and  his  contemporaries  should  have  ex- 
pected to  find  such  a  region  of  intersection,  as  it  implied  that 
the  explosion  had  occurred  within  a  few  thousand  years.  The 
fact  that  the  required  conditions  were  not  fulfilled  was  no  argu- 
ment against  the  hypothesis,  because  the  explosion  might  have 
occurred  millions  of  years  ago ;  and  in  the  mean  time  the  peri- 
helion and  node  of  each  orbit  would  have  made  many  entire 
revolutions,  so  that  the  orbits  would  have  been  completely 
mixed  up.  ...  A  different  explanation  of  the  group  is  given 
by  the  nebular  hypothesis ;  so  that  Olbers's  hypothesis  is  no 
longer  considered  by  astronomers." 

247.  Later  Discoveries  of  Asteroids.  —  Since  1845  over 
two  hundred  asteroids  have  been  discovered.  All  these  are 
so  small,  that  it  requires  a  very  good  telescope  to  see  them ; 
and  even  in  very  powerful  telescopes  they  appear  as  mere 
points  of  light,  which  can  be  distinguished  from  the  stars 
only  by  their  motions. 


ASTRONOMY.  243 

To  facilitate  the  discovery  of  these  bodies,  very  accurate 
maps  have  been  constructed,  including  all  the  stars  down  to 
the  thirteenth  magnitude  in  the  neighborhood  of  the  ecliptic. 
A  reduced  copy  of  one  of  these  maps  is  shown  in  Fig.  271. 

Furnished  with  a  map  of  this  kind,  and  with  a  telescope 
powerful  enough  to  show  all  the  stars  marked  on  it,  the 


Fig.  271. 

observer  who  is  searching  for  these  small  planets  will  place 
in  the  field  of  view  of  his  telescope  six  spider-lines  at  right 
angles  to  each  other,  and  at  equal  distances  apart,  in  such 
a  manner  that  several  small  squares  will  be  formed,  embra- 
cing just  as  much  of  the  heavens  as  do  those  shown  in  the 
map.  He  will  then  direct  his  telescope  to  the  region  of  the 
sky  he  wishes  to  examine,  represented  by  the  map,  so  as  to 
be  able  to  compare  successively  each  square  with  the  corre- 


244 


ASTRONOMY. 


spending  portion  of  the  sky.  Fig.  272  shows  at  the  right 
hand  the  squares  in  the  telescopic  field  of  view,  and  at  the 
left  hand  the  corresponding  squares  of  the  map. 

He  can  then  assure  himself  if  the  numbers  and  positions  of 
the  stars  mapped,  and  of  the  stars  observed,  are  identical.  If 
he  observes  in  the  field  of  view  a  luminous  point  which  is  not 
marked  in  the  map,  it  is  evident  that  either  the  new  body  is  a 
star  of  variable  brightness  which  was  not  visible  at  the  time 


Fig.  272. 

the  map  was  made,  or  it  is  a  planet,  or  perhaps  a  comet.  If 
the  new  body  remains  fixed  at  the  same  point,  it  is  the  former: 
but,  if  it  changes  its  position  with  regard  to  the  neighboring 
stars,  it  is  the  latter.  The  motion  is  generally  so  sensible,  that 
in  the  course  of  one  evening  the  change  of  position  may  be 
detected ;  and  it  can  soon  be  determined,  by  the  direction  and 
rate  of  the  motion,  whether  the  body  is  a  planet  or  a  comet. 

IV.     OUTER   GROUP   OF   PLANETS. 

JUPITER. 

248.  Orbit  of  Jupiter. — The  orbit  of  Jupiter  is  inclined 
only  a  little  over  one  degree  to  the  ecliptic  ;  and  its  eccen- 
tricity is  only  about  half  of  that  of  Mars,  being  less  than 
one-twentieth.  The  mean  distance  of  Jupiter  from  the  sun 
is  about  four  hundred  and  eighty  million  miles ;  but,  owing 
to  the  eccentricity  of  his  orbit,  his  actual  distance  from  the 
sun  ranges  from  four  hundred  and  fifty-seven  to  five  hun- 
dred and  three  million  miles. 


ASTRONOMY.  245 

249.  Distance  of  Jupiter  from  the  Earth.  — _When 
Jupiter  is  in  opposition,  his  mean  distance  from  the  earth 
is  four  hundred  and  eighty  million  miles  minus  ninety-two 
million  miles,  or  three  hundred  and  eighty-eight  million 
miles,  and,  when  he  is  in  conjunction,  four  hundred  and 
eighty  million  miles  plus  ninety-two  million  miles,  or  five 
hundred  and  seventy-two  million  miles.  It  will  be  seen 
that  he  is  less  than  twice  as  far  off  in  conjunction  as  in 
opposition,  and  that  the  ratio  of  his  greatest  to  his  least 
distance  is  very  much  less  than  in  the  case  of  Venus  and 
Mars.  This  is  owing  to  his  very  much  greater  distance  from 
the  sun.  Owing  to  the  eccentricities  of  the  orbits  of  the 


Fig.    273. 

earth  and  of  Jupiter,  the  greatest  and  least  distances  of 
Jupiter  from  the  earth  vary  somewhat  from  year  to  year. 

250.  The  Brightness  and  Apparent  Size  of  Jupiter.  — 
The   apparent  diameter  of  Jupiter  varies  from  about  fifty 
seconds  to  about  thirty  seconds.     His  apparent  size  at  his 
extreme  and  mean  distances  from  the  earth  is  shown  in 
Fig.  273. 

Jupiter  shines  with  a  brilliant  white  light,  which  exceeds 
that  of  every  other  planet  except  Venus.  The  planet  is, 
of  course,  brightest  when  near  opposition. 

251.  The  Volume  and  Density  of  Jupiter.  —  Jupiter  is 
the  "  giant  planet "  of  our  system,  his  mass  largely  exceed- 
ing  that   of  all   the   other  planets   combined.     His   mean 


246 


ASTRONOMY, 


diameter  is  about  eighty-five  thousand  miles ;  but  the  equa- 
torial exceeds  the  polar  diameter  by  five  thousand  miles. 
In  volume  he  exceeds  our  earth  about  thirteen  hundred 
times,  but  in  mass  only  about  two  hundred  and  thirteen 
times.  His  specific  gravity  is,  therefore,  far  less  than  that 
of  the  earth,  and  even  less  than  that  of  water.  The  com- 
parative size  of  Jupiter  and  the  earth  is  shown  in  Fig.  274. 

252.   The  Sidereal  and  Sy nodical  Periods  of  Jupiter. — 
It  takes  Jupiter  nearly  twelve  years  to  make  a  sidereal  revo- 


Fig.  274. 

lution,  or  a  complete  revolution  around  the  sun,  his  orbital 
motion  being  at  the  rate  of  about  eight  miles  a  second. 
His  synodical  period,  or  the  time  of  his  passage  from  oppo- 
sition to  opposition  again,  is  three  hundred  and  ninety-eight 
days. 

253.  The  Telescopic  Aspect  of  Jupiter.  —  There  are  no 
really  permanent  markings  on  the  disk  of  Jupiter ;  but  his 
surface  presents  a  very  diversified  appearance.  The  earlier 
telescopic  observers  descried  dark  belts  across  it,  one  north 
of  the  equator, 'and  the  other  south  of  it.  With  the  in- 
crease of  telescopic  power,  it  was  seen  that  these  bands 


PJATEV, 


ASTRONOMY.  247 

were  of  a  more  complex  structure  than  had  been  supposed, 
and  consisted  of  stratified,  cloud-like  appearances,  varying 
greatly  in  form  and  number.  These  change  so  rapidly,  that 
the  face  of  the  planet  rarely  presents  the  same  appearance 
on  two  successive  nights.  They  are  most  strongly  marked 
at  some  distance  on  each  side  of  the  planet's  equator,  and 
thus  appear  as  two  belts  under  a  low  magnifying  power. 

Both  the  outlines  of  the  belts,  and  the  color  of  portions 
of  the  planet,  are  subject  to  considerable  changes.  The 
equatorial  regions,  and  the  spaces  between  the  belts  gener- 
ally, are  often  of  a  rosy  tinge.  This  color  is  sometimes 
strongly  marked,  while  at  other  times  hardly  a  trace  of  it 
can  be  seen.  A  general  telescopic  view  of  Jupiter  is  given 
in  Plate  V. 

254.  The  Physical  Constitution  of  Jupiter.  —  From  the 
changeability  of  the  belts,  and  of  nearly  all  the  visible 
features  of  Jupiter,  it  is  clear  that  what  we  see  on  that 
planet  is  not  the  solid  nucleus,  but  cloud-like  formations, 
which  cover  the  entire  surface  to  a  great  depth.  The  planet 
appears  to  be  covered  with  a  deep  and  dense  atmosphere, 
filled  with  thick  masses  of  clouds  and  vapor.  Until  recently 
this  cloud-laden  atmosphere  was  supposed  to  be  somewhat 
like  that  of  our  globe ;  but  at  present  the  physical  constitu- 
tion of  Jupiter  is  believed  to  resemble  that  of  the  sun  rather 
than  that  of  the  earth.  Like  the  sun,  he  is  brighter  in  the 
centre  than  near  the  edges,  as  is  shown  in  the  transits  of 
the  satellites  over  his  disk.  When  the  satellite  first  enters 
on  the  disk,  it  commonly  seems  like  a  bright  spot  on  a 
dark  background ;  but,  as  it  approaches  the  centre,  it 
appears  like  a  dark  spot  on  the  bright  surface  of  the 
planet.  The  centre  is  probably  two  or  three  times  brighter 
than  the  edges.  This  may  be,  as  in  the  case  of  the  sun, 
because  the  light  near  the  edge  passes  through  a  greater 
depth  of  atmosphere,  and  is  diminished  by  absorption. 

It  has  also  been  suspected  that  Jupiter  shines  partly  by 


248 


ASTRONOMY. 


his  own  light,  and  not  wholly  by  reflected  sunlight.  The 
planet  cannot,  however,  emit  any  great  amount  of  light ; 
for,  if  it  did,  the  satellites  would  shine  by  this  light  when 
they  are  in  the  shadow  of  the  planet,  whereas  they  to- 
tally disappear.  It  is  possible  that  the  brighter  portions 
of  the  surface  are  from  time  to  time  slightly  self-lumi- 
nous. 

Again :  the  interior  of  Jupiter  seems  to  be  the  seat  of  an 
activity  so  enormous  that  it  can  be  ascribed  only  to  intense 


Fig.  275. 

heat.  Rapid  movements  are  always  occurring  on  his  sur- 
face, often  changing  its  aspect  in  a  few  hours.  It  is  there- 
fore probable  that  Jupiter  is  not  yet  covered  by  a  solid 
crust,  and  that  the  fiery  interior,  whether  liquid  or  gaseous, 
is  surrounded  by  the  dense  vapors  which  cease  to  be  lumi- 
nous on  rising  into  the  higher  and  cooler  regions  of  the 
atmosphere.  Figs.  275  and  276  show  the  disk  of  Jupiter 
as  it  appeared  in  December,  1881. 

255.  Rotation  of  Jupiter.  —  Spots  are  sometimes  visible 


ASTRONOMY. 


249 


which  are  much  more  permanent  than  the  ordinary  mark- 
ings on  the  belts.     The  most  remarkable  of  these  is  "  the 


Fig.  276. 

great  red  spot,"  which  was  first  observed  in  July,  1878, 
and  is  still  to  be  seen  in  February,  1882.  It  is  shown 
just  above  the  centre  of  the  disk  in  Fig.  275.  By  watch- 


Fig.  277. 

ing  these  spots  from  day  to  day,  the  time  of  Jupiter's  axial 
rotation  has  been  found  to  be  about  nine  hours  and  fifty 
minutes. 


250 


ASTRONOMY. 


The  axis  of  Jupiter  deviates  but  slightly  from  a  perpen- 
dicular to  the  plane  of  its  orbit,  as  is  shown  in  Fig.  277. 

THE  SATELLITES  OF  JUPITER. 

256.   Jupiter's  Four  Moons.  —  Jupiter  is  accompanied 


Fig.  278. 

by  four  moons,  as  shown  in  Fig.  278.  The  diameters  of 
these  moons  range  from  about  twenty-two  hundred  to  thirty- 
seven  hundred  miles.  The  second  from  the  planet  is  the 
smallest,  and  the  third  the  largest.  The  smallest  is  about 
the  size  of  our  moon ;  the  largest  considerably  exceeds 


Fig.  279. 

Mercury,  and  almost  rivals  Mars,  in  bulk.  The  sizes  of 
these  moons,  compared  with  those  of  the  earth  and  its 
moon,  are  shown  in  Fig.  279. 

The  names  of  these  satellites,  in  the  order  of  their  dis- 
tance from  the  planet,  are  Io,  Europa,  Ganymede,  and  Cal- 


ASTRONOMY.  25 1 

Us  to.  Their  times  of  revolution  range  from  about  a  day 
and  three-fourths  up  to  about  sixteen  days  and  a  half. 
Their  orbits  are  shown  in  Fig.  280. 

257.  The  Variability  of  Jupiter's  Satellites.  —  Remarka- 
ble variations  in  the  light  of  these  moons  have  led  to  the 
supposition  that  violent  changes  are  taking  place  on  their 
surfaces.  It  was  formerly  believed,  that,  like  our  moon, 


Fig.  280. 

they  always  present  the  same  face  to  the  planet,  and  that 
the  changes  in  their  brilliancy  are  due  to  differences  in  the 
luminosity  of  parts  of  their  surface  which  are  successively 
turned  towards  us  during  a  revolution ;  but  careful  measure- 
ments of  their  light  show  that  this  hypothesis  does  not 
account  for  the  changes,  which  are  sometimes  very  sudden. 
The  satellites  are  too  distant  for  examination  of  their  sur- 
faces with  the  telescope  :  hence  it  is  impossible  to  give  any 
certain  explanation  of  these  phenomena. 


252 


ASTRONOMY. 


258.  Eclipses  of  Jupiter's  Satellites. — Jupiter,  like  the 
earth,   casts   a   shadow   away  from   the    sun,  as   shown    in 


Fig.  281. 

Fig.  281  ;  and,  whenever  one  of  his  moons  passes  into  this 
shadow,  it  becomes  eclipsed.     On  the  other  hand,  whenever 


ASTRONOMY. 


253 


one  of  the  moons  throws  its  shadow  on  Jupiter,  the  sun  is 
eclipsed  to  that  part  of  the  planet  which  lies  within  the 
shadow. 

To  the  inhabitants  of  Jupiter   (if  there  are  any,  and  if 


Fig.  282. 

they  can  see  through  the  clouds)  these  eclipses  must  be 
very  familiar  affairs  ;  for  in  consequence  of  the  small  incli- 
nations of  the  orbits  of  the  satellites  to  the  planet's  equator, 
and  the  small  inclination  of  the  latter  to  the  plane  of 
Jupiter's  orbit,  all  the  satellites,  except  the  most  distant  one, 


254  ASTRONOMY. 

are  eclipsed  in  every  revolution.  A  spectator  on  Jupiter 
might  therefore  witness  during  the  planetary  year  forty-five 
hundred  eclipses  of  the  moons,  and  about  the  same  number 
of  the  sun. 

259.  Transits  of  Jupiter's  Satellites.  —  Whenever  one 
of  Jupiter's  moons  passes  in  front  of  the  planet,  it  is  said 
to  make  a  transit  across  his  disk.  When  a  moon  is  making 
a  transit,  it  presents  its  bright  hemisphere  towards  the  earth, 
as  will  be  seen  from  Fig.  282  :  hence  it  is  usually  seen  as  a 
bright  spot  on  the  planet's  disk ;  though  sometimes,  on  the 
brighter  central  portions  of  the  disk,  it  appears  dark. 


Fig.  283. 

It  will  be  seen  from  Fig.  282  that  the  shadow  of  a  moon 
does  not  fall  upon  the  part  of  the  planet's  disk  that  is 
covered  by  the  moon  :  hence  we  may  observe  the  transit 
of  both  the  moon  and  its  shadow.  The  shadow  appears 
as  a  small  black  spot,  which  will  precede  or  follow  the 
moon  according  to  the  position  of  the  earth  in  its  orbit. 
Fig.  283  shows  two  moons  of  Jupiter  in  transit. 

260.  Occultations  of  Jupiter's  Satellites. — The  eclipse 
of  a  moon  of  Jupiter  must  be  carefully  distinguished  from 
the  occupation  of  a  moon  by  the  planet.  In  the  case  of 
an  eclipse,  the  moon  ceases  to  be  visible,  because  the  mass 


ASTRONOMY.  255 

of  Jupiter  is  interposed  between  the  sun  and  the  moon, 
which  ceases  to  be  luminous,  because  the  sun's  light  is  cut 
off;  but,  in  the  case  of  an  occultation,  the  moon  gets  into 
such  a  position  that  the  body  of  Jupiter  is  interposed  be- 
tween it  and  the  earth,  thus  rendering  the  moon  invisible 
to  us.  The  third  satellite,  m"  (Fig.  282),  is  invisible  from 
the  earth  E,  having  become  occulted  when  it  passed  behind 
the  planet's  disk;  but 
it  will  not  be  eclipsed 
until  it  passes  into  the 
shadow  of  Jupiter. 

261.  Jupiter  without 
Satellites,  —  It  occasion- 
ally happens  that  every 
one    of    Jupiter's    satel- 
lites  will    disappear    at 
the    same    time,    either 
by    being    eclipsed     or 
occulted,  or  by  being  in 
transit.     In    this    event, 
Jupiter  will  appear  with- 
out satellites.     This  oc- 
curred  on  the   2ist  of 

August,  1867.     The  po-  Fig.  284. 

sition    of    Jupiter's    satellites    at    this    time    is    shown    in 

Fig.  284. 

SATURN. 
THE  PLANET  AND  HIS  MOON'S. 

262.  The    Orbit   of  Saturn.  —  The    orbit   of    Saturn   is 
rather  more  eccentric  than  that  of  Jupiter,  its  eccentricity 
being  somewhat  more  than   one-twentieth.     Its   inclination 
to  the  ecliptic  is  about  two  degrees  and  a  half.     The  mean 
distance  of  Saturn  from  the  sun  is  about  eight  hundred  and 
eighty  million  miles.     It  is  about  a  hundred  million  miles 
nearer  the  sun  at  perihelion  than  at  aphelion. 


256  ASTRONOMY. 

263.  Distance  of  Saturn  from  the  Earth. — The  mean 
distance  of  Saturn  from  the  earth  at  opposition  is  eight  hun- 
dred  and    eighty  million    miles    minus   ninety-two   million 
miles,  or  seven  hundred  and  eighty-eight  million  ;  and  at 
conjunction,  eight  hundred  and  eighty  million  miles  plus 
ninety-two  million,  or  nine  hundred  and  seventy-two  million. 
Owing  to  the  eccentricity  of  the  orbit  of  Saturn,  his  dis- 
tance from  the  earth  at  opposition  and  at  conjunction  varies 
by  about  a  hundred  million  miles  at  different  times ;  but  he 
is  so  immensely  far  away,  that  this  is  only  a  small  fraction 
of  his  mean  distance. 

264.  Apparent  Size   and  Brightness   of  Satitrn. —  The 
apparent  diameter  of  Saturn  varies  from  about  twenty  sec- 
onds to  about  fourteen  seconds.     His  apparent  size  at  his 


Fig.  285. 

extreme  and  mean   distances   from  the   earth  is  shown  in 
Fig.  285. 

The  planet  generally  shines  with  the  brilliancy  of  a  mod- 
erate first-magnitude  star,  and  with  a  dingy,  reddish  light, 
as  if  seen  through  a  smoky  atmosphere. 

265.  Volume  and  Density  of  Saturn.  —  The  real  diame- 
ter  of  Saturn    is    about    seventy    thousand   miles,    and    its 
volume  over  seven  hundred  times   that  of  the  earth.     The 
comparative  size  of  the  earth  and  Saturn  is  shown  in  Fig. 
286.     This  planet   is   a  little   more  than  half  as  dense  as 
Jupiter. 

266.  The  Sidereal  and  Synodical  Periods  of  Saturn. — 
Saturn   makes  a  complete  revolution  round  the   sun  in  a 
period  of  about  twenty-nine   years  and  a  half,  moving  in 
his  orbit  at  the  rate  of  about  six  miles   a   second.     The 


ASTRONOMY. 


257 


planet  passes  from  opposition  to  opposition  again  in  a 
period  of  three  hundred  and  seventy-eight  days,  or  thirteen 
days  over  a  year. 

267.  Physical  Constitution  of  Saturn.  —  The  physical 
constitution  of  Saturn  seems  to  resemble  that  of  Jupiter; 
but,  being  twice  as  far  away,  the  planet  cannot  be  so  well 
studied.  The  farther  an  object  is  from  the  sun,  the  less  it 
is  illuminated  ;  and,  the  farther  it  is  from  .the  earth,  the 


Fig.  286. 

smaller  it  appears  :  hence  there  is  a  double  difficulty  in 
examining  the  more  distant  planets.  Under  favorable  cir- 
cumstances, the  surface  of  Saturn  is  seen  to  be  diversified 
with  very  faint  markings ;  and,  with  high  telescopic  powers, 
two  (  or  more  very  faint  streaks,  or  belts,  may  be  discerned 
parallel  to  its  equator.  These  belts,  like  those  of  Jupiter, 
change  their  aspect  from  time  to  time ;  but  they  are  so  faint 
that  the  changes  cannot  be  easily  followed.  It  is  only  on 
rare  occasions  that  the  time  of  rotation  can  be  determined 
from  a  study  of  the  markings. 


258  ASTRONOMY. 

268.  Rotation  of  Saturn.  —  On  the  evening  of  Dec.  7, 
1876,  Professor  Hall,  who  had  been  observing  the  satellites 
of  Saturn  with  the  great  Washington  telescope  (18),  saw  a 
brilliant  white  spot  near  the  equator  of  the  planet.  It 
seemed  as  if  an  immense  eruption  of  incandescent  matter 
had  suddenly  burst  up  from  the  interior.  The  spot  gradu- 
ally spread  itself  out  into  a  long  light  streak,  of  which  the 
brightest  point  was  near  the  western  end.  It  remained  visi- 
ble until  January,  when  it  became  faint  and  ill-defined,  and 
the  planet  was  lost  in  the  rays  of  the  sun. 


Fig.  287. 

From  all  the  observations  on  this  spot,  Professor  Hall 
found  the  period  of  Saturn  to  be  ten  hours  fourteen  minutes, 
reckoning  by  the  brightest  part  of  the  streak.  Had  the 
middle  of  the  streak  been  taken,  the  time  would  have  been 
less,  because  the  bright  matter  seemed  to  be  carried  along 
in  the  direction  of  the  planet's  rotation.  If  this  motion 
was  due  to  a  wind,  the  velocity  of  the  current  must  have 
been  between  fifty  and  a  hundred  miles  an  hour.  The  axis 
of  Saturn  is  inclined  twenty-seven  degrees  from  the  per- 
pendicular to  its  orbit. 


ASTRONOMY. 


259 


269.  The  Satellites  of  Saturn.  —  Saturn  is  accompanied 
by  eight  moons.  Seven  of  these  are  shown  in  Fig.  287. 
The  names  of  these  satellites,  in  the  order  of  their  distances 
from  the  planet,  are  given  in  the  accompanying  table  :  — 


Distance 

1 

NAME. 

from 
Planet  in 

Sidereal  Period. 

Discoverer. 

Date  of  Discovery. 

55 

Miles. 

d.  h.  m. 

d. 

I 

Mimas     .     . 

120,800 

o  22  37 

0.94 

Herschel.     . 

Sept.    17,   1789. 

2 

Enceladus    . 

iSS.ooo 

i     853 

i-37 

Herschel  .     . 

Aug.    28,   1789. 

3 

Tethys     .     . 

191,900 

I    21     l8 

1.88 

Cassini     .     . 

March,       1684. 

4 

Dione  .     .     . 

245,800 

2    17    41 

2-73 

Cassini     .     . 

March,       1684. 

5 

Rhea  .     .     . 

343»4oo 

4  I2  25 

4-51 

Cassini     .     . 

Dec.    23,   1672. 

6 

Titan  .     .     . 

796,100 

15    22    41 

15-94 

Huyghens    . 

March  25,  1655. 

7 

Hyperion 

963>30° 

21     7     7 

21.29 

Bond  .     .     . 

Sept.    16,    1848. 

8 

Japetus    .     . 

2,313,800 

79     7  53 

79-33 

Cassini     .     . 

October,     1671. 

The  apparent  brightness  or  visibility  of  these  satellites 
follows  the  order  of  their  discovery.  The  smallest  telescope 
will  show  Titan,  and  one  of  very  moderate  size  will  show 
Japetus  in  the  western  part  of  its  orbit.  An  instrument  of 
four  or  five  inches  aperture  will  show  Rhea,  and  perhaps 
Tethys  and  Dione ;  while  seven  or  eight  inches  are  required 
for  Enceladus,  even  at  its  greatest  elongation  from  the  planet. 


Fig.  288. 

Mimas  can  rarely  be  seen  except  at  its  greatest  elongation,  and 
then  only  with  an  aperture  of  twelve  inches  or  more.  Hype- 
rion can  be  detected  only  with  the  most  powerful  telescopes, 
on  account  of  its  faintness  and  the  difficulty  of  distinguishing 
it  from  minute  stars. 

Japetus,  the  outermost  satellite,  is  remarkable  for  the  fact, 
that  while,  in  one  part  of  its  orbit,  it  is  the  brightest  of  the 
satellites  except  Titan,  in  the  opposite  part  it  is  almost  as 


260 


ASTRONOMY. 


Fig.  289. 


ASTRONOMY.  26 1 

faint  as  Hyperion,  and  can  be  seen  only  in  large  telescopes. 
When  west  of  the  planet,  it  is  bright ;  when  east  of  it,  faint. 
This  peculiarity  has  been  accounted  for  by  supposing  that  the 
satellite,  like  our  moon,  always  presents  the  -same  face  to  the 
planet,  and  that  one  side  of  it  is  white  and  the  other  intensely 
black;  but  it  is  doubtful  whether  any  known  substance  is  so 
black  as  one  side  of  the  satellite  must  be  to  account  for  such 
extraordinary  changes  of  brilliancy. 

Titan,  the   largest   of  these   satellites,  is  about  the  size 
of  the   largest  satellite  of  Jupiter.     The  relative  sizes  of 


Fig.  290. 

the  satellites  are   shown  in  Fig.   288,   and   their   orbits   in 
Fig.  289. 

Fig.  290  shows  the  transit  of  one  of  the  satellites,  and 
of  its  shadow,  across  the  disk  of  the  planet. 

THE  RINGS   OF  SATURN. 

270.  General  Appearance  of  the  Rings.  —  Saturn  is  sur- 
rounded by  a  thin  flat  ring  lying  in  the  plane  of  its  equator. 
This  ring  is  probably  less  than  a  hundred  miles  thick.  The 
part  of  it  nearest  Saturn  reflects  little  sunlight  to  us ;  so  that 
it  has  a  dusky  appearance,  and  is  not  easily  seen,  although 
it  is  not  quite  so  dark  as  the  sky  seen  between  it  and  the 
planet.  The  outer  edge  of  this  dusky  portion  of  the  ring 
is  at  a  distance  from  Saturn  of  between  two  and  three 
times  the  earth's  diameter.  Outside  of  this  dusky  part  of 


262 


ASTRONOMY. 


Fig.  291. 


ASTRONOMY.  263 

the  ring  is  a  much  brighter  portion,  and  outside  of  this 
another,  which  is  somewhat  fainter,  but  still  so  much  brighter 
than  the  dusky  part  as  to  be  easily  seen.  The  width  of  the 
brighter  parts  of  the  ring  is  over  three  times  the  earth's 
diameter.  To  distinguish  the  parts,  the  outer  one  is  called 
ring  A,  the  middle  one  ring  B,  and  the  dusky  one  ring  C. 
Between  A  and  B  is  an  apparently  open  space,  nearly  two 
thousand  miles  wide,  which  looks  like  a  black  line  on  the 
ring.  -  Other  divisions  in  the  ring  have  been  noticed  at 
times ;  but  this  is  the  only  one  always  seen  with  good  tele- 
scopes at  times  when  either  side  of  the  ring  is  in  view  from 
the  earth.  The  general  telescopic  appearance  of  the  ring 
is  shown  in  Fig.  291. 


Fig.  292. 

Fig.  292  shows  the  divisions  of  the  rings  as  they  were 
seen  by  Bond. 

271.  Phases  of  Saturn's  Ring. — The  ring  is  inclined 
to  the  plane  of  the  planet's  orbit  by  an  angle  of  twenty- 
seven  degrees.  The  general  aspect  from  the  earth  is  nearly 
the  same  as  from  the  sun.  As  the  planet  revolves  around 
the  sun,  the  axis  and  plane  of  the  ring  keep  the  same 
direction  in  space,  just  as  the  axis  of  the  earth  and  the 
plane  of  the  equator  do. 

When  the  planet  is  in  one  part  of  its  orbit,  we  see  the 


264 


ASTRONOMY. 


upper  or  northern  side  of  the  ring  at  an  inclination  of 
twenty-seven  degrees,  the  greatest  angle  at  which  the  ring 
can  ever  be  seen.  This  phase  of  the  ring  is  shown  in 
Fig.  293. 


Fig.  293. 

When  the  planet  has  moved  through  a  quarter  of  a 
revolution,  the  edge  of  the  ring  is  turned  towards  the  sun 
and  the  earth;  and,  owing  to  its  extreme  thinness,  it  is 
visible  only  in  the  most  powerful  telescopes  as  a  fine  line 


Fig.  294. 

of  light,  stretching  out  on  each  side  of  the  planet.     This 
phase  of  the  ring  is  shown  in  Fig.  294. 

All  the  satellites,  except  Japetus,  revolve  very  nearly  in 
the  plane  of  the  ring :  consequently,  when  the  edge  of  the 
ring  is  turned  towards  the  earth,  the  satellites  seem  to  swing 


ASTRONOMY. 


265 


from  one  side  of  the  planet  to  the  other  in  a  straight  line, 
running  along  the  thin  edge  of  the  ring  like  beads  on  a 
string.  This  phase  affords  the  best  opportunity  of  seeing 


Fig.  295. 

the   inner  satellites,  Mimas  and  Enceladus,  which  at  other 
times  are  obscured  by  the  brilliancy  of  the  ring. 


Fig.  296. 

Fig.  295  shows  a  phase  of  the  ring  intermediate  between 
the  last  two. 

When  the   planet  has  moved  ninety  degrees  farther,  we 


266 


ASTRONOMY. 


again  see  the  ring  at  an  angle  of  twenty-seven  degrees ;  but 
now  it  is  the  lower  or  southern  side  which  is  visible.  When 
it  has  moved  ninety  degrees  farther,  the  edge  of  the  ring 
is  again  turned  towards  the  earth  and  sun. 

The  successive  phases  of  Saturn's  ring  during  a  complete 
revolution  are  shown  in  Fig.  296. 


Fig.  297. 

It  will  be  seen  that  there  are  two  opposite  points  of 
Saturn's  orbit  in  which  the  rings  are  turned  edgewise  to 
us,  and  two  points  half-way  between  the  former  in  which 
the  ring  is  seen  at  its  maximum  inclination  of  about  twenty- 
seven  degrees.  Since  the  planet  performs  a  revolution  in 
twenty-nine  years  and  a  half,  these  phases  occur  at  average 
intervals  of  about  seven  years  and  four  months. 


ASTRONOMY.  2.6j 

272.  Disappearance  of  Saturn's  Ring.  —  It  will  be  seen 
from  Fig.  297  that  the  plane  of  the  ring  may  not  be  turned 
towards  the  sun  and  the  earth  at  exactly  the  same  time,  and 
also  that  the  earth  may  sometimes  come  on  one  side  of  the 
plane  of  the  ring  while  the  sun  is  shining  on  the  other.  In 
the  figure,  E,  E' ,  E",  and  E'"  is  the  orbit  of  the  earth. 
When  Saturn  is  at  61',  or  opposite,  at  /%  the  plane  of  the  ring 
will  pass  through  the  sun,  and  then  only  the  edge  of  the  ring 
will  be  illumined.  Were  Saturn  at  S,  and  the  earth  at  E\  the 
plane  of  the  ring  would  pass  through  the  earth.  This  would 
also  be  the  case  were  the  earth  at  E'",  and  Saturn  at  S". 


Fig.  298. 

Were  Saturn  at  6*  or  at  S",  and  the  earth  farther  to  the  left  or 
to  the  right,  the  sun  would  be  shining  on  one  side  of  the  ring 
while  we  should  be  looking  on  the  other.  In  all  these  cases 
the  ring  will  disappear  entirely  in  a  telescope  of  ordinary 
power.  With  very  powerful  telescopes  the  ring  will  appear,  in 
the  first  two  cases,  as  a  thin  line  of  light  (Fig.  298).  It  will 
be  seen  that  all  these  cases  of  disappearance  must  take  place 
when  Saturn  is  in  the  parts  of  his  orbit  intercepted  between 
the  parallel  lines  A  C  and  BD.  These  lines  are  tangent  to 
the  earth's  orbit,  which  they  enclose,  and  are  parallel  to  the 
plane  of  Saturn's  ring.  As  Saturn  passes  away  from  these 
two  lines  on  either  side,  the  rings  appear  more  and  more  open. 
When  the  dark  side  of  the  ring  is  in  view,  it  appears  as  a 


268  ASTRONOMY. 

black  line  crossing  the  planet ;  and  on  such  occasions  the  sun- 
light reflected  from  the  outer  and  inner  edges  of  the  rings  A 
and  B  enables  us  to  see  traces  of  the  ring  on  each  side  of 
Saturn,  at  least  in  places  where  two  such  reflections  come 
nearly  together.  Fig.  299  illustrates  this  reflection  from  the 
edges  at  the  divisions  of  the  rings. 

273.  Changes  in  Saturn 's  Ring.  —  The  question  whether 
changes  are  going  on  in  the  rings  of  Saturn  is  still  unsettled. 
Some  observers  have  believed  that  they  saw  additional  divis- 
ions in  the  rings  from  time  to  time;  but  these  may  have  been 
errors  of  vision,  due  partly  to  the  shading  which  is  known  to 
exist  on  portions  of  the  ring. 


Fig.  299. 

Professor  Newcomb  says,  '•  As  seen  with  the  great  Wash- 
ington equatorial  in  the  autumn  of  1874,  there  was  no  great  or 
sudden  contrast  between  the  inner  or  dark  edge  of  the  bright 
ring  and  the  outer  edge  of  the  dusky  ring.  There  was  some 
suspicion  that  the  one  shaded  into  the  other  by  insensible 
gradations.  No  one  could  for  a  moment  suppose,  as  some 
observers  have,  that  there  was  a  separation  between  these  two 
rings.  All  these  considerations  give  rise  to  the  question 
whether  the  dusky  ring  may  not  be  growing  at  the  expense 
of  the  inner  bright  ring." 

Struve,  in  1851,  advanced  the  startling  theory  that  the  inner 
edge  of  the  ring  was  gradually  approaching  the  planet,  the 


ASTRONOMY.  269 

whole  ring  spreading  inwards,  and  making  the  central  opening 
smaller.  The  theory  was  based  upon  the  descriptions  and 
drawings  of  the  rings  by  the  astronomers  of  the  seventeenth 
century,  especially  Huyghens,  and  the  measures  made  by  later 
astronomers  up  to  1851.  This  supposed  change  in  the  dimen- 
sion of  the  ring  is  shown  in  Fig.  300. 

274.  Constitution  of  Saturn's  Ring.  —  The  theory  now  gen- 
erally held  by  astronomers  is,  that  the  ring  is  composed  of  a 
cloud  of  satellites  too  small  to  be  separately  seen  in  the  tele- 
scope, and  too  close  together  to  admit  of  visible  intervals 
between  them.  The  ring  looks  solid,  because  its  parts  are 
too  small  and  too  numerous  to  be  seen  singly.  They  are  like 
the  minute  drops  of  water  that  make  up  clouds  and  fogs, 
which  to  our  eyes  seem  like  solid  masses.  In  the  dusky  ring 
the  particles  may  be  so  scattered  that  we  can  see  through 


Fig.  300. 

the  cloud,  the  duskiness  being  clue  to  the  blending  of  light  and 
darkness.  Some  believe,  however,  that  the  duskiness  is  caused 
by  the  darker  color  of  the  particles  rather  than  by  their  being 
farther  apart. 

URANUS. 

275.  Orbit  and  Dimensions  of  Uranus.  —  Uranus,  the 
smallest  of  the  outer  group  of  planets,  has  a  diameter  of 
nearly  thirty-two  thousand  miles.  It  is  a  little  less  dense 
than  Jupiter,  and  its  mean  distance  from  the  sun  is  about 
seventeen  hundred  and  seventy  millions  of  miles.  Its  orbit 
has  about  the  same  eccentricity  as  that  of  Jupiter,  and  is 
inclined  less  than  a  degree  to  the  ecliptic.  Uranus  makes 


27O  ASTRONOMY. 

a  revolution  around  the  sun  in  eighty-four  years,  moving  at 
the  rate  of  a  little  over  four  miles  a  second.  It  is  visible 
to  the  naked  eye  as  a  star  of  the  sixth  magnitude. 

As  seen  in  a  large  telescope,  the  planet  has  a  decidedly 
sea-green  color ;  but  no  markings  have  with  certainty  been 

detected  on  its  disk,  so  that 
nothing  is  really  known  with 
regard  to  its  rotation.  Fig. 
301  shows  the  comparative 
size  of  Uranus  and  the 
earth. 

276.  Discovery  of  Uranus. 
Fig.  301-  —  This  planet  was  discovered 

by  Sir  William  Herschel  in  March,  1781.  He  was  engaged 
at  the  time  in  examining  the  small  stars  of  the  constellation 
Gemini,  or  the  Twins.  He  noticed  that  this  object  which 
had  attracted  his  attention  had  an  appreciable  disk,  and 
therefore  could  not  be  a  star.  He  also  perceived  by  its 
motion  that  it  could  not  be 
a  nebula  ;  he  therefore  con- 
cluded that  it  was  a  comet, 
and  announced  his  discovery 
as  such.  On  attempting  to 
compute  its  orbit,  it  was 
soon  found  that  its  motions 
could  be  accounted  for  only 
on  the  supposition  that  it 
was  moving  in  a  circular 

orbit  at  about  twice  the  dis-    

tance    of    Saturn   from   the  Fis-  3°2- 

sun.  It  was  therefore  recognized  as  a  new  planet,  whose 
discovery  nearly  doubled  the  dimensions  of  the  solar  system 
as  it  was  then  known. 

277.    The  Name  of  the  Planet.  —  Herschel,  out   of   compli 
merit   to   his  patron,  George  III.,  proposed   to   call   the   new 


ASTRONOMY.  27! 

planet  Georgium  Sidus  (the  Georgian  Star);  but  this  name 
found  little  favor.  The  name  of  Herschel  was  proposed,  and 
continued  in  use  in  England  for  a  time,  but  did  not  meet  with 
general  approval.  Various  other  names  were  suggested,  and 
finally  that  of  Uranus  was  adopted. 

278.  The  Satellites  of  Uranus.  —  Uranus  is  accompanied 
by  four  satellites,  whose  orbits  are  shown  in  Fig..  302.     These 
satellites  are  remarkable  for  the  great  inclination  of  their 
orbits  to  the  plane  of  the  planet's  orbit,  amounting  to  about 
eighty  degrees,  and   for  their  retrograde  motion ;    that  is, 
they  move  from  east  to  west,  instead  of  from  west  to  east, 
as  in  the  case  of  all  the  planets  and  of  all  the  satellites 
previously  discovered. 

NEPTUNE. 

279.  Orbit    and  Dimensions    of  Neptune.  —  So    far  as 
known,  Neptune  is  the  most  remote  member  of  the  solar 
system,  its  mean  distance  from  the  sun  being  twenty-seven 
hundred  and  seventy-five  million  miles.     This  distance  is 
considerably   less   than    twice    that    of   Uranus.      Neptune 
revolves  around  the   sun  in  a  period  of  a  little  less  than 
a  hundred  and  sixty-five  years.     Its  orbit  has  but   slight 
eccentricity,  and  is  inclined  less  than  two  degrees  to  the 
ecliptic.     This   planet   is  considerably  larger  than  Uranus, 
its  diameter  being  nearly  thirty-five  thousand  miles.     It  is 
somewhat  less  dense  than  Uranus.     Neptune  is  invisible  to 
the  naked  eye,  and  no  telescope  has  revealed  any  markings 
on  its  disk :    hence  nothing  is   certainly  known   as   to   its 
rotation.     Fig.  303  shows  the  comparative  size  of  Neptune 
and  the  earth. 

280.  The    Discovery    of   Neptune.  —  The    discovery   of 
Neptune  was  made  in  1846,  and  is  justly  regarded  as  one 
of  the  grandest  triumphs  of  astronomy. 

Soon  after  Uranus  was  discovered,  certain  irregularities  in 
its  motion  were  observed,  which  could  not  be  explained.     It 


2/2  ASTRONOMY. 

is  well  known  that  the  planets  are  all  the  while  disturbing 
each  other's  motions,  so  that  none  of  them  describe  perfect 
ellipses.  These  mutual  disturbances  are  called  perturba- 
tions. In  the  case  of  Uranus  it  was  found,  that,  after 
making  due  allowance  for  the  action  of  all  the  known 
planets,  there  were  still  certain  perturbations  in  its  course 
which  had  not  been  accounted  for.  This  led  astronomers 
to  the  suspicion  that  these  might  be  caused  by  an  unknown 
planet.  Leverrier  in  France,  and  Adams  in  England,  inde- 
pendently of  each  other,  set  themselves  the  difficult  problem 
of  computing  the  position  and  magnitude  of  a  planet  which 
would  produce  these  perturbations.  Both,  by  a  most  labo- 
rious computation,  showed  that  the 
perturbations  were  such  as  would  be 
produced  by  a  planet  revolving  about 
the  sun  at  about  twice  the  distance 
of  Uranus,  and  having  a  mass  some- 
what greater  than  that  of  this  planet ; 
and  both  pointed  out  the  same  part 
3°3-  of  the  heavens  as  that  in  which  the 

planet  ought  to  be  found  at  that  time.  Almost  immedi- 
ately after  they  had  announced  the  conclusion  to  which 
they  had  arrived,  the  planet  was  found  with  the  tele- 
scope. The  astronomer  who  was  searching  for  the  planet 
at  the  suggestion  of  Leverrier  was  the  first  to  recognize 
it :  hence  Leverrier  has  obtained  the  chief  credit  of  the 
discovery. 

The  observed  planet  is  proved  to  be  nearer  than  the  one 
predicted  by  Leverrier  and  Adams,  and  therefore  of  smaller 
magnitude. 

281.  The  Observed  Planet  not  the  Predicted  One.— Pro- 
fessor  Peirce  always  maintained  that  the  planet  found  by  obser- 
vation was  not  the  one  whose  existence  had  been  predicted  by 
Leverrier  and  Adams,  though  its  action  would  completely  ex- 
plain all  the  irregularities  in  the  motion  of  Uranus.  His  last 


ASTRONOMY. 


273 


statement  on  this  point  is  as  follows  :  "  My  position  is,  that 
there  were  two  possible  planets,  either  of  which  might  have 
caused  the  observed  irregular 
motions  of  Uranus.  %  Each 
planet  excluded  the  other;  so 
that,  if  one  was,  the  other  was 
not.  They  coincided  in  direc- 
tion from  the  earth  at  certain 
epochs,  once  in  six  hundred 
and  fifty  years.  It  was  at  one 
of  these  epochs  that  the  pre- 
diction was  made,  and  at  no 
other  time  for  six  centuries 
could  the  prediction  of  the 
one  planet  have  revealed  the  Fi§-  sm- 

other.    The  observed  planet  was  not  the  predicted  one." 

282.  B ode's  Law  Disproved. — The  following  table  gives 
the  distances  of  the  planets  according  to  Bode's  law,  their 
actual  distances,  and  the  error  of  the  law  in  each  case  :  — 


PLANET. 

Numbers  of  Bode. 

Actual 
Distances. 

Errors. 

Mercury      .... 
Venus    

o  4-  4  =      4 
i  4-  4  —      ? 

3-9 

7  2 

O.I 

O  2 

Earth     

6  ~\~  4  —    10 

IO  O 

O  O 

Mars  

i"  +  4  —    16 

I  S-** 

0.8 

Minor  planets      .     . 
Jupiter  

24  +  4=    28 
j.8  ~f~  j.  —    c'' 

20  to  35 

C"7  O 

o  o 

Saturn    

96  ~\~  4  —  TOO 

jj_.vj 

QC   A 

4.6 

Uranus  

1  0^    -\-   A    —    I  Q6 

IQI.Q 

4.1 

Neptune     .... 

384  +  4  =  388 

300  6 

87.4 

It  will  be  seen,  that,  before  the  discovery  of  Neptune,  the 
agreement  was  so  close  as  to  indicate  that  this  was  an  actual 
law  of  the  distances ;  but  the  discovery  of  this  planet  com- 
pletely disproved  its  existence. 


2/4  ASTRONOMY. 

283.  The  Satellite   of  Neptune.  —  Neptune  is  accompa- 
nied by  at  least  one  moon,  whose  orbit  is  shown  in  Fig. 
304.     The  orbit  of  this   satellite    is    inclined   about   thirty 
degrees  to  the  plane  of  the  ecliptic,  and  the  motion  of  the 
satellite  is  retrograde,  or  from  east  to  west. 

VII.     COMETS   AND   METEORS. 

I.     COMETS. 
GENERAL  PHENOMENA  OF  COMETS. 

284.  General  Appearance  of  a  Bright  Comet.  —  Comets 
bright  enough  to  be  seen  with  the  naked  eye  are  composed 
of  three    parts,  which    run    into    each    other  by  insensible 

gradations.  These 
are  the  nucleus,  the 
coma,  and  the  tail. 

The  nucleus  is  the 
bright  centre  of  the 
comet,  and  appears 
to  the  eye  as  a 
star  or  planet. 

The  coma  is  a 
nebulous  mass  sur- 
rounding the  nu- 
cleus on  all  sides.. 
Close  to  the  nucleus 
it  is  almost  as  bright 
as  the  nucleus  it- 
self; but  it  gradually 

shades  off  in  every  direction.  The  nucleus  and  coma  com- 
bined appear  like  a  star  shining  through  a  small  patch  of 
fog ;  and  these  two  together  form  what  is  called  the  head 
of  the  comet. 

The  tail  is  a  continuation  of  the  coma,  and  consists  of  a 


ASTRONOMY. 


275 


stream    of    milky   light,   growing   wider   and   fainter   as   it 
recedes  from  the  head,  till  the  eye  is  unable  to  trace  it. 

The  general  appearance  of  one  of  the   smaller  of  the 
brilliant  comets  is  shown  in  Fig.  305. 


Fig.  306. 

285.  General  Appearance  of  a  Telescopic  Comet.  —  The 
great  majority  of  comets  are  too  faint  to  be  visible  with  the 
naked  eye,  and  are  called 
telescopic  comets.  In  these 
comets  there  seems  to  be 
a  development  of  coma 
at  the  expense  of  nucleus 
and  tail.  In  some '  cases 
the  telescope  fails  to  re- 
veal any  nucleus  at  all  in 
one  of  these  comets ;  at 
other  times  the  nucleus  is 
so  faint  and  ill-defined  as 
to  be  barely  distinguisha- 
ble. Fig.  306  shows  a 
telescopic  comet  without 
any  nucleus  at  all,  and  Fis-  3°7- 

another  with  a  slight  condensation  at  the  centre.  In  these 
comets  it  is  generally  impossible  to  distinguish  the  coma 
from  the  tail,  the  latter  being  either  entirely  invisible,  as  in 


2/6  ASTRONOMY. 

Fig.  306,  or  else  only  an  elongation  of  the  coma,  as  shown 


Fig.  308. 

in  Fig.  307.     Many  comets  appear  simply  as   patches   of 


Fig.  309. 

foggy  light  of  more  or  less  irregular  form. 


ASTRONOMY. 


277 


286.  The  Development  of  Telescopic  Comets  on  their 
Approach  to  the  Sun.  —  As  a  rule,  all  comets  look  nearly 
alike  when  they  first  come  within  the  reach  of  the  tele- 
scope. They  appear  at  first  as  little  foggy  patches,  with- 
out any  tail,  and  often  with- 
out any  visible  nucleus.  As 
they  approach  the  sun  their 
peculiarities  are  rapidly  de- 
veloped. Fig.  308  shows 
such  a  comet  as  first  seen, 
and  the  gradual  development 
of  its  nucleus,  head,  and 
tail,  as  it  approaches  the 
sun. 

If    the    comet    is    only   a 
small  one,  the  tail  developed  Fig-  310- 

is  small ;  but  these  small  appendages  have  a  great  variety 
of  form  in  different  comets.  Fig.  309  shows  the  singular 
form  into  which  Encke's  comet  was  developed  in  1871. 

Figs.  310  and  311  show 
other  peculiar  developments 
of  telescopic  comets. 

287.  Development  of  Bril- 
liant Comets  on  their  Ap- 
proach to  the  Sun.  —  Bril- 
liant comets,  as  well  as 
telescopic  comets,  appear 
nearly  alike  when  they  come 
into  the  view  of  the  tele- 
scope ;  and  it  is  only  on 
Flg-  3"-  their  approach  to  the  sun 

that  their  distinctive  features  are  developed.  Not  only  do 
these  comets,  when  they  first  come  into  view,  resemble 
each  other,  but  they  also  bear  a  close  resemblance  to  tele- 
scopic comets. 


278 


ASTRONOMY. 


As  the  comet  approaches  the  sun,  bright  vaporous  jets, 
two  or  three  in  number,  are  emitted  from  the  nucleus  on 
the  side  of  the  sun  and  in  the  direction  of  the  sun.  These 
jets,  though  directed  towards  the  sun,  are  soon  more  or  less 
carried  backward,  as  if  repelled  by  the  sun.  Fig.  312 
shows  a  succession  of  views  of  these  jets  as  they  were 
developed  in  the  case  of  Halley's  comet  in  1835. 


Fig.  312. 

The  jets  in  this  case  seemed  to  have  an  oscillatory  motion. 
At  i  and  2  they  seemed  to  be  attracted  towards  the  sun, 
and  in  3  to  be  repelled  by  him.  In  4  and  5  they  seemed 
to  be  again  attracted,  and  in  6  to  be  repelled,  but  in  a 
reverse  direction  to  that  in  3.  In  7  they  appeared  to  be 
again  attracted.  Bessel  likened  this  oscillation  of  the  jets 
to  the  vibration  of  a  magnetic  needle  when  presented  to 
the  pole  of  a  magnet. 

In  the  case  of  larger  comets  these  luminous  jets    are  sur- 


ASTRONOMY.  2/9 

rounded  by  one  or  more  envelops,  which  are  thrown  off  in 
succession  as  the  comet  approaches  the  sun.  The  forma- 
tion of  these  envelops  was  a  conspicuous  feature  of  Donates 
comet  of  1858.  A  rough  view  of  the  jets  and  the  surround- 
ing envelops  is  given  in  Fig.  313.  Fig.  314  gives  a  view 
of  the  envelops  without  the  jets. 

288.  The  Tails  of  Comets.  —  The  tails  of  brilliant  comets 
are  rapidly  formed  as  the  comet  approaches  the  sun,  their 
increase  in  length  often 
being  at  the  rate  of  several 
million  miles  a  day.  These 
appendages  seem  to  be 
formed  entirely  out  of  the 
matter  which  is  emitted 
from  the  nucleus  in  the 
luminous  jets  which  are 
at  first  directed  towards 
the  sun.  The  tails  of 
comets  are,  however,  always 
directed  away  from  the  sun, 
as  shown  in  Fig.  315. 

It  will  be  seen  that  the 
comet,  as  it  approaches  the 
sun,  travels  head  foremost ; 
but  as  it  leaves  the  sun  it 
goes  tail  foremost.  Fis-  3*3- 

The  apparent  length  of  the  tail  of  a  comet  depends 
partly  upon  its  real  length,  partly  upon  the  distance  of  the 
comet,  and  partly  upon  the  direction  of  the  axis  of  the  tail 
with  reference  to  the  line  of  vision.  The  longer  the  tail, 
the  nearer  the  comet ;  and  the  more  nearly  at  right  angles 
to  the  line  of  vision  is  the  axis  of  the  tail,  the  greater  is 
the  apparent  length  of  the  tail.  In  the  majority  of  cases 
the  tails  of  comets  measure  only  a  few  degrees ;  but,  in  the 
case  of  many  comets  recorded  in  history,  the  tail  has  ex- 
tended half  way  across  the  heavens. 


28O  ASTRONOMY. 

The  tail  of  a  comet,  when  seen  at  all,  is  usually  several 
million  miles  in  length ;  and  in  some  instances  the  tail  is 
long  enough  to  reach  across  the  orbit  of  the  earth,  or  twice 
as  far  as  from  the  earth  to  the  sun. 

The  tails  of  comets  are  apparently  hollow,  and  are  some- 
times a  million  of  miles  in  diameter.  So  great,  however, 
is  the  tenuity  of  the  matter  in  them,  that  the  faintest  stars 


are  seen  through  it  without  any  apparent  obscuration.     See 
Fig.  316,  which  is  a  view  of  the  great  comet  of  1264. 

The  tails  of  comets  are  sometimes  straight,  as  in  Fig. 
316,  but  usually  more  or  less  curved,  as  in  Fig.  317,  which 
is  a  view  of  Donates  comet  as  it  appeared  at  one  time. 
The  tail  of  a  comet  is  occasionally  divided  into  a  number 
of  streamers,  as  in  Figs.  318  and  319.  Fig.  318  is  a 
view  of  the  great  comet  of  1744,  and  Fig.  319  of  the 


ASTRONOMY. 


28l 


great  comet  of  1861.  No.  i,  in  Fig.  320,  is  a  view  of  the 
comet  of  1577 ;  No.  2,  of  the  comet  of  1680;  and  No.  3, 
of  the  comet  of  1769. 

Fig.  321  shows  some  of  the 
forms  which  the  imagination 
of  a  superstitious  age  saw  de- 
picted in  comets,  when  these 
heavenly  visitants  were  thought 
to  be  the  forerunners  of  wars, 
pestilence,  famine,  and  other 
dire  calamities. 

289.  Visibility  of  Comets. — 
Even  the  brightest  comets  are 
visible  only  a  short  time  near 
their  perihelion  passage.  When 
near  the  sun,  they  sometimes 
become  very  brilliant,  and  on 
rare  occasions  have  been  visible 
even  at  mid-day.  It  is  seldom 
that  a  comet  can  be  seen,  even 
with  a  powerful  telescope,  dur- 
ing its  perihelion  passage,  un- 
less its  perihelion  is  either  in- 
side of  the  earth's  orbit,  or 
but  little  outside  of  it. 


Fig.  315. 


MOTION  AND  ORIGIN  OF  COMETS. 

290.  Recognition  of  a  Telescopic  Comet.  —  It  is  impossi- 
ble to  distinguish  telescopic  comets  by  their  appearance 
from  another  class  of  heavenly  bodies  known  as  nebulce. 
Such  comets  can  be  recognized  only  by  their  motion. 


282  ASTRONOMY. 

Thus,  in  Fig.  322,  the  upper  and  lower  bodies  look  exactly 
alike ;  but  the  upper  one  is  found  to  remain  stationary, 
while  the  lower  one  moves  across  the  field  of  view.  The 
upper  one  is  thus  shown  to  be  a  nebula,  and  the  lower 
one  a  comet. 

291.  Orbits  of  Comets.  —  All  comets  are  found  to  move 
in  very  eccentric  ellipses,  in  parabolas,  or  in  hyperbolas. 

Since  an  ellipse  is  a 
closed  curve  (48),  all  com- 
ets that  move  in  ellipses, 
no  matter  how  eccentric, 
are  permanent  members 
of  the  solar  system,  and 
will  return  to  the  sun  at 
intervals  of  greater  or  less 
length,  according  to  the 
size  of  the  ellipses  and 
the  rate  of  the  comet's 
motion. 

Parabolas  and  hyperbo- 
las being  open  curves  (48), 
comets  that  move  in  either 
of   these    orbits    are    only 
temporary  members  of  our 
solar  system.     After  pass- 
Fig.  316.  ing   the    sun,    they   move 
off  into  space,  never  to  return,  unless  deflected  hither  by 
the  action  of  some  heavenly  body  which  they  pass  in  their 
journey. 

Since  a  comet  is  visible  only  while  it  is  near  the  sun,  it  is 
impossible  to  tell,  by  the  form  of  the  portion  of  the  orbit 
which  it  describes  during  the  period  of  its  visibility,  whether 
it  is  a  part  of  a  very  elongated  ellipse,  a  parabola,  or  a  hyper- 
bola. Thus  in  Fig.  323  are  shown  two  orbits,  one  of  which 
is  a  very  elongated  ellipse,  and  the  other  a  parabola.  The 


ASTRONOMY. 


Fig.  317- 

part  ab,  in   each  case,  is  the  portion  of  the  orbit  described 
by  the  comet  during  its  visibility.     While  describing  the  dotted 


Fig.  318. 

portions  of  the  orbit,  the  comet  is  invisible.     Now  it  is  impos- 


284 


ASTRONOMY. 


sible  to  distinguish  the  form  of  the  visible  portion  in  the 
two  orbits.  The  same  would  be  true  were  one  of  the  orbits 
a  hyperbola. 

Whether  a  comet  will  describe  an  ellipse,  a  parabola,  or  a 
hyperbola,  can  be  determined  only  by  its  velocity,  taken  in  con- 
nection with  its  distance  from  the  sun.  Were  a  comet  ninety- 
two  and  a  half  million  miles  from  the  sun,  moving  away  from 
the  sun  at  the  rate  of  twenty-six  miles  a  second,  it  would  have 
iust  the  velocity  necessary  to  describe  a  parabola.  Were  it 
moving  with  a  greater  velocity,  it  would  necessarily  describe 


Fig.  319. 

a  hyperbola,  and,  with  a  less  velocity,  an  ellipse.  So,  at  any 
distance  from  the  sun,  there  is  a  certain  velocity  which  would 
cause  a  comet  to  describe  a  parabola;  while  a  greater  velocity 
would  cause  it  to  describe  a  hyperbola,  and  a  less  velocity  to 
describe  an  ellipse.  If  the  comet  is  moving  in  an  ellipse,  the 
less  its  velocity,  the  less  the  eccentricity  of  its  orbit :  hence,  in 
order  to  determine  the  form  of  the  orbit  of  any  comet,  it  is 
only  necessary  to  ascertain  its  distance  from  the  sun,  and  its 
velocity  at  any  given  time. 


ASTRONOMY. 


28S 


Fig.  320. 


Comets  move  in  every  direction  in  their  orbits,  and  these 
orbits  have  every  conceivable  inclination  to  the  ecliptic. 


286 


ASTRONOMY. 


11 


Fig.  321. 

292.  Periodic   Comets.  —  There    are   quite  a   number  of 


ASTRONOMY. 


287 


comets  which  are  known  to  be  periodic,  returning  to  the 
sun  at  regular  intervals  in  elliptic  orbits.  Some  of  these 
have  been  observed  at  sev- 
eral returns,  so  that  their 
period  has  been  determined 
with  great  certainty.  In  the 
case  of  others  the  perio- 
dicity is  inferred  from  the 
fact  that  the  velocity  fell 
so  far  short  of  the  parabolic 
limit  that  the  comet  must 
move  in  an  ellipse.  The 
number  of  known  periodic 
comets  is  increasing  every  Fig.  322. 

year,  three  having  been  added  to  the  list  in  1881. 

The  velocity  of  most  comets  is  so  near  the  parabolic  limit 
that  it  is  not  possible  to  decide,  from  observations,  whether  it 
falls  short  of  it,  or  exceeds  it.  In  the  case  of  a  few  comets 

the  observations  indi- 
cate a  minute  excess  of 
velocity ;  but  this  cannot 
be  confidently  asserted. 
It  is  not,  therefore,  abso- 
lutely certain  that  any 
known  comet  revolves 
in  a  hyperbolic  orbit; 
and  thus  it  is  possible 
that  all  comets  belong 
to  our  system,  and  will 
ultimately  return  to  it. 
It  is,  however,  certain, 
that,  in  the  majority  of 
cases,  the  return  will  be 
delayed  for  many  cen- 
turies, and  perhaps  for  many  thousand  years. 

293.  Origin  of  Comets.  —  It  is  now  generally  believed  that 
the  original  home  of  the  comets  is  in  the  stellar  spaces  outside 


\\ 


Fig.  323. 


288  ASTRONOMY. 

of  our  solar  system,  and  that  they  are  drawn  towards  the  sun, 
one  by  one,  "in  the  long  lapse  of  ages.  Were  the  sun  unaccom- 
panied by  planets,  or  were  the  planets  immovable,  a  comet 
thus  drawn  in  would  whirl  around  the  sun  in  a  parabolic  orbit, 
and  leave  it  again  never  to  return,  unless  its  path  were  again 
deflected  by  its  approach  to  some  star.  But,  when  a  comet  is 
moving  in  a  parabola,  the  slightest  retardation  would  change 
its  orbit  to  an  ellipse,  and  the  slightest  acceleration  into  a 
hyperbola.  Owing  to  the  motion  of  the  several  planets  in 
their  orbits,  the  velocity  of  a  comet  would  be  changed  on 
passing  each  of  them.  Whether  its  velocity  would  be  acceler- 
ated or  retarded,  would  depend  upon  the  way  in  which  it  passed. 
Were  the  comet  accelerated  by  the  action  of  the  planets,  on 
its  passage  through  our  system,  more  than  it  was  retarded  by 
them,  it  would  leave  the  system  with  a  more  than  parabolic 
orbit,  and  would  therefore  move  in  a  hyperbola.  Were  it,  on 
the  contrary,  retarded  more  than  accelerated  by  the  action  of 
the  planets,  its  velocity  would  be  reduced,  so  that  the  comet 
would  move  in  a  more  or  less  elongated  ellipse,  and  thus 
become  a  permanent  member  of  the -solar  system. 

In  the  majority  of  cases  the  retardation  would  be  so  slight 
that  it  could  not  be  detected  by  the  most  delicate  observation, 
and  the  comet  would  return  to  the  sun  only  after  the  expiration 
of  tens  or  hundreds  of  thousands  of  years;  but,  were  the 
comet  to  pass  very  near  one  of  the  larger  planets,  the  retarda- 
tion might  be  sufficient  to  cause  the  comet  to  revolve  in  an 
elliptical  orbit  of  quite  a  short  period.  The  orbit  of  a'  comet 
thus  captured  by  a  planet  would  have  its  aphelion  point  near 
the  orbit  of  the  planet  which  captured  it.  Now.  it  happens 
that  each  of  the  larger  planets  has  a  family  of  comets  whose 
aphelia  are  about  its  own  distance  from  the  sun.  It  is  there- 
fore probable  that  these  comets  have  been  captured  by  the 
action  of  these  planets.  As  might  be  expected  from  the  gigan- 
tic size  of  Jupiter,  the  Jovian  family  of  comets  is  the  largest. 
The  orbits  of  several  of  the  comets  of  this  group  are  shown 
in  Fig.  324. 

294.  Number  of  Comets.  —  The  number  of  comets  re- 
corded as  visible  to  the  naked  eye  since  the  birth  of  Christ 


ASTRONOMY.  289 

is  about  five  hundred,  while  about  two  hundred  telescopic 
comets  have  been  observed  since  the  invention  of  the  tele- 
scope. The  total  number  of  comets  observed  since  the 
Christian  era  is  therefore  about  seven  hundred.  It  is  cer- 
tain, however,  that  only  an  insignificant  fraction  of  all  exist- 
ing comets  have  ever  been  observed.  Since  they  can  be 


Fig.  324. 

seen  only  when  near  their  perihelion,  and  since  it  is  probable 
that  the  period  of  most  of  those  which  have  been  observed 
is  reckoned  by  thousands  of  years  (if,  indeed,  they  ever 
return  at  all),  our  observations  must  be  continued  for  many 
thousand  years  before  we  have  seen  all  which  come  within 
range  of  our  telescopes.  Besides,  as  already  stated  (289), 
a  comet  can  seldom  be  seen  unless  its  perihelion  is  either 


2QO  ASTRONOMY. 

inside  the  orbit  of  the  earth,  or  but  little  outside  of  it ;  and 
it  is  probable  that  the  perihelia  of  the  great  majority  of 
comets  are  beyond  this  limit  of  visibility. 

REMARKABLE  COMETS. 

295.  The  Comet  of  1680.  —  The  great  comet  of  1680,  shown 
in  Fig.  320,  is  one  of  the  most  celebrated  on  record.     It  was 
by  his  study  of  its  motions  that  Newton  proved  the  orbit  of 
a  comet  to  be  one  of   the  conic  sections,  and  therefore  that 
these  bodies  move  under  the  influence  of  gravity.     This  comet 
descended  almost  in  a  direct  line  to  the  sun,  passing  nearer  to 
that   luminary  than   any  comet  before  known.     Newton  esti- 
mated, that,  at  its  perihelion  point,  it  was  exposed  to  a  tempera- 
ture   two   thousand   times   that   of    red-hot  iron.     During  its 
perihelion  passage  it  was   exceedingly  brilliant.     Halley  sus- 
pected  that   this    comet   had   a   period   of    five   hundred   and 
seventy-five  years,  and  that  its  first  recorded  appearance  was 
in  43  B.C.,  its  third  in  1106,  and  its  fourth  in   1680.     If   this  is 
its  real  period,  it  will  return  in  2255.     The  comet  of  43  B.C. 
made   its   appearance    just   after   the   assassination   of    Julius 
Caesar.     The  Romans  called  it  the  Julian  Star,  and  regarded 
it  as  a  celestial  chariot  sent  to  convey  the  soul  of  Caesar  to  the 
skies.     It  was  seen  two  or  three  hours  before  sunset,  and  con- 
tinued visible  for  eight  successive  days.     The  great  comet  of 
no6was  described  as  an  object  of  terrific  splendor,  and  was 
visible  in  close  proximity  to  the  sun.     The  comet  of  1680  has 
become  celebrated,  not  only  on  account  of  its  great  brilliance, 
and  on  account  of  Newton's  investigation  of  its  orbit,  but  also 
on  account  of   the  speculation  of  the  theologian  Whiston  in 
regard  to  it.     He  accepted  five  hundred  and  seventy-five  years 
as  its  period,  and  calculated  that  one  of  its  earlier  apparitions 
must  have  occurred  at  the  date  of  the  flood,  which  he  supposed 
to  have  been  caused  by  its  near  approach  to  the  earth ;  and  he 
imagined  that  the  earth  is  doomed  to  be  destroyed  by  fire  on 
some  future  encounter  with  this  comet. 

296.  The  Comet  of  1811.  —  The  great  comet  of  1811,  a  view 
of  which  is  given  in  Fig.  325,  is,  perhaps,  the  most  remarkable 
comet  on  record.     It  was  visible  for  nearly  seventeen  months, 


ASTRONOMY. 


29I 


and  was  very  brilliant,  although  at  its  perihelion  passage  it 
was  over  a  hundred  million  miles  from  the  sun.  Its  tail  was 
a  hundred  and  twenty  million  miles  in  length,  and  several 
million  miles  through.  It  has  been  calculated  that  its  aphelion 
point  is  about  two  hundred  times  as  far  from  the  sun  as  its 
perihelion  point,  or  some  seven  times  the  distance  of  Neptune 
from  the  sun.  Its  period  is  estimated  at  about  three  thousand 
years.  It  was  an  object  of  superstitious  terror,  especially  in 
the  East.  The  Russians  regarded  it  as  presaging  Napoleon's 
great  and  fatal  war  with  Russia. 


Fig.  325. 

297.  Halley's  Comet.  —  Halley's  comet  has  become  one  of 
the  most  celebrated  of  modern  times.  It  is  the  first  comet 
whose  return  was  both  predicted  and  observed.  It  made  its 
appearance  in  1682.  Halley  computed  its  orbit,  and  compared 
it  with  those  of  previous  comets,  whose  orbits  he  also  com- 
puted from  recorded  observations.  He  found  that  it  coincided 
so  exactly  with  that  of  the  comet  observed  by  Kepler  in  1607, 
that  there  could  be  no  doubt  of  the  identity  of  the  two  orbits. 
So  close  were  they  together,  that,  were  they  both  drawn  in  the 


292 


ASTRONOMY. 


heavens,  the  naked  eye  would  almost  see  them  joined  into  one 
line.  There  could  therefore  be  no  doubt  that  the  comet  of 
1682  was  the  same  that  had  appeared  in  1607,  and  that  it  moved 
in  an  elliptic  orbit,  with  a  period  of  about  seventy-five  years. 

He  found  that  this 
comet  had  previ- 
ously appeared  in 
1531  and  in  1456; 
and  he  predicted 
that  it  would  return 
about  1758.  Its 
actual  return  was 
retarded  somewhat 
by  the  action  of 
the  planets  on  it  in 
its  passage  through 
the  solar  system. 
It,  however,  ap- 
Fis-  326.  peared  again  in 

1759,  and  a  third  time  in  1835.  Its  next  appearance  will  be 
about  1911.  The  orbit  of  this  comet  is  shown  in  Fig.  326. 
Fig.  327  shows  the  comet  as  it  appeared  to  the  naked  eye,  and 


Fig.  327. 


in  a  telescope  of  moderate  power,  in  1835.  This  comet  appears 
to  be  growing  less  brilliant.  In  1456  it  appeared  as  a  comet 
of  great  splendor ;  and  coming  as  it  did  in  a  very  superstitious 
age,  soon  after  the  fall  of  Constantinople,  and  during  the  threat- 


ASTRONOMY. 


ened  invasion  of  Europe  by  the  Turks,  it  caused  great  alarm. 
Fig.  328  shows  the  changes  undergone  by  the  nucleus  of  this 
comet  during  its  perihelion  passage  in  1835. 

298.  Encke'  s  Comet.  —  This  telescopic  comet,  two  views  of 
which  are  given  in  Figs.  329  and  330,  appeared  in  1818.  Encke 
computed  its  orbit,  and  found  it  to  lie  wholly  within  the  orbit 
of  Jupiter  (Fig.  324),  and  the  period  to  be  about  three  years 
and  a  third.  By  comparing  the  intervals  between  the  succes- 


Fig.  328. 

sive  returns  of  this  comet,  it  has  been  ascertained  that  its 
orbit  is  continually  growing  smaller  and  smaller.  To  account 
for  the  retardation  of  this  comet,  Olbers  announced  his  cele- 
'brated  hypothesis,  that  the  celestial  spaces  are  filled  with  a 
subtile  resisting  medium.  This  hypothesis  was  adopted  by 
Encke,  and  has  been  accepted  by  certain  other  astronomers ; 
but  it  has  by  no  means  gained  universal  assent. 

299.  Bieln's  Comet. —  This  comet  appeared  in  1826,  and 
was  found  to  have  a  period  of  about  six  years  and  two  thirds. 
On  its  return  in  1845,  it  met  with  a  singular,  and  as  yet  unex- 


2Q4  ASTRONOMY. 

plained,  accident,   which   has   rendered   the   otherwise   rather 

insignificant  comet  famous.     In  November  and  December  of 

that  year  it  was  observed  as  usual,  without  any  thing  remarka- 
ble about  it;  but,  in 
January  of  the  fol- 
lowing year,  it  was 
found  to  have  been 
divided  into  two  dis- 
tinct parts,  so  as  to 
appear  as  two  comets 
instead  of  one.  The 
two  parts  were  at 
first  of  very  unequal 
brightness ;  but,  dur- 
ing the  following 
month,  the  smaller 
329-  of  the  two  increased 

in    brilliancy   until   it    equalled    its    companion ;    it   then   grew 

fainter  till  it  entirely  disappeared,  a  month  before  its  companion. 

The  two  parts  were  about  two  hundred  thousand  miles  apart. 

Fig.  331  shows  these 

two    parts     as    they 

appeared  on  the  I9th 

of  February,  and  Fig. 

332  as  they  appeared 

on  the  2  ist  of  Feb- 
ruary. On  its  return 

in  1852,    the    comets 

were    found    still    to 

be   double ;    but  the 

two  components  were 

now  about  a  million 

and      a     half     miles 

apart.        They      are 

shown    in    Fig.    333 

as  they  appeared  at  Fig.  330. 

this  time.     Sometimes  one  of  the  parts  appeared  the  brighter, 

and  sometimes  the  other;  so  that  it  was  impossible  to  decide 

which  was  really  the  principal  comet.     The  two  portions  passed 


ASTRONOMY.  2Q5 

out  of  view  in  September,  and  have  not  been  seen  since; 
although  in  1872  the  position  of  the  comet  would  have  been 
especially  favorable  for  observation.  The  comet  appears  to 
have  become  completely  broken  up. 


Fig.  33i. 

300.  The  Comet  of  1843.  —  The  great  comet  of  1843,  a  view 
of  which  is  given  in  Fig.  334,  was  favorably  situated  for  obser- 
vation only  in  southern  latitudes.  It  was  exceedingly  brilliant, 


Fig.  332. 

and  was  easily  seen  in  full  daylight,  in  close  proximity  to  the 
sun.  The  apparent  length  of  its  tail  was  sixty-five  degrees, 
and -its  real  length  a  hundred  and  fifty  million  miles,  or  nearly 


296  ASTRONOMY. 

twice  the  distance  from  the  earth  to  the  sun.  This  comet  is 
especially  remarkable  on  account  of  its  near  approach  to  the 
sun.  At  the  time  of  its  perihelion  passage  the  distance  of 
the  comet  from  the  photosphere  of  the  sun  was  less  than 
one-fourteenth  of  the  diameter  of  the  sun.  This  distance  was 
only  one-half  that  of  the  comet  of  1680  when  at  its  perihelion. 
When  at  perihelion,  this  comet  was  plunging  through  the  sun's 
outer  atmosphere  at  the  rate  of  one  million,  two  hundred  and 
eighty  thousand  miles  an  hour.  It  passed  half  way  round  the 
sun  in  the  space  of  two  hours,  and  its  tail  was  whirled  round 
through  a  hundred  and  eighty  degrees  in  that  brief  time.  As 


Fig-  333- 

the  tail  extended  almost  double  the  earth's  distance  from  the 
sun,  the  end  of  the  tail  must  have  traversed  in  two  hours  a 
space  nearly  equal  to  the  circumference  of  the  earth's  orbit, — 
a  distance  which  the  earth,  moving  at  the  rate  of  about  twenty 
miles  a  second,  is  a  whole  year  in  passing.  It  is  almost  impos- 
sible to  suppose  that  the  matter  forming  this  tail  remained  the 
same  throughout  this  tremendous  sweep. 

301.  Donates  Comet.  —  The  great  comet  of  1858.  known  as 
Donates  comet,  was  one  of  the  most  magnificent  of  modern 
times.  When  at  its  brightest  it  was  only  about  fifty  million 
miles  from  the  earth.  Its  tail  was  then  more  than  fifty  mil- 
lion miles  long.  Had  the  comet  at  this  time  been  directly 
between  the  earth  and  sun,  the  earth  must  have  passed  through 


ASTRONOMY.  297 

its  tail ;  but  this  did  not  occur.     The  orbit  of  this  comet  was 

found  to  be  decidedly  elliptic,  with  a  period  of  about  two  thou. 

sand  years.     This  comet  is  especially  celebrated  on  account 

of  the  careful  telescopic  observations  of  its  nucleus  and  coma 

at  the  time  of  its  perihelion  passage.     Attention  has  already 

been    called    (287)    to    the    changes    it 

underwent   at   that    time.     Its   tail   was 

curved,   and    of    a    curious   feather-like 

form,  as  shown  in  Fig.  335.     At  times 

it  developed  lateral  streamers,  as  shown 

in  Fig.  336.     Fig.  337  shows  the  head 

of  the  comet   as    it  was  seen  by  Bond 

of     the     Harvard     Observatory,    whose 

delineations   of    this    comet    have    been 

justly  celebrated. 

302.  The  Comet  of  1861.  —  The  great 
comet    of    1861    is    remarkable    for   its 
great    brilliancy,    for    its    peculiar    fan- 
shaped  tail,   and  for  the    probable   pas- 
sage of  the  earth  through  its  tail.     Sir 
John   Herschel  declared  that   it    far  ex- 
ceeded in   brilliancy  any  comet  he   had 
ever  seen,  not  excepting  those  of  1811 
and  1858.     Secchi  found  its   tail    to    be 
a     hundred    and    eighteen    degrees    in 
length,   the    largest  but  one   on   record. 
Fig.  338  shows  this  comet  as  it  appeared 
at  one  time.     Fig.  339  shows  the   posi- 
tion  of   the    earth  at   E.   in   the   tail  of 
this  comet,  on   the  3oth  of  June,    1861. 
Fig.  340  shows  the  probable  passage  of 
the  earth  through  the  tail  of  the  comet 
on  that  date.     As    the    tail    of  a  comet 
doubtless    consists    of   something  much 

less  dense  than  our  atmosphere,  it  is  not  surprising  that  no 
noticeable  effect  was  produced  upon  us  by  the  encounter,  if  it 
occurred. 

303.  Coggia^s  Comet.  —  This  comet,  which  appeared  in  1874, 
looked  very  large,  because  it  came  very  near  the  earth.     It  was 


298 


ASTRONOMY. 


Fig-  335. 

not  at  all  brilliant.     Its  nucleus  was  carefully  studied,  and  was 


Fig.  336. 


ASTRONOMY.  299 

found    to   develop   a   series   of  envelops   similar  to   those  of 
Donati's  comet.     Figs.  341  and  342  are  two  views  of  the  head 


Fig.  337- 

of   this  comet.     Fig.  343  shows  the  system  of  envelops  that 
were  developed  during  its  perihelion  passage. 


3OO  ASTRONOMY. 

304.   The  Comet  of  June,  1881.  —  This    comet,  though   far 
from  being  one  of  the  largest  of  modern  times,  was  still  very 


Fig.  338. 

brilliant.  It  will  ever  be  memorable  as  the  first  brilliant  comet 
which  has  admitted  of  careful  examination  with  the  spec- 
troscope. 


Fig-  339- 

CONNECTION  BETWEEN  METEORS  AND  COMETS. 

305.  Sho o ting- Star s .  —  On    watching    the    heavens    any 
clear  night,  we  frequently  see  an  appearance  as  of  a  star 


ASTRONOMY.  30 1 

shooting  rapidly  through  a  short  space  in  the  sky,  and  then 
suddenly  disappearing.  Three  or  four  such  shooting-stars 
may,  on  the  average,  be  observed  in  the  course  of  an  hour., 
They  are  usually  seen  only  a  second  or  two  ;  but  they  some- 
times move  slowly,  and  are  visible  much  longer.  These 
stars  begin  to  be  visible  at  an  average  height  of  about 
seventy-five  miles,  and  they  disappear  at  an  average  height 
of  about  fifty  miles.  They  are  occasionally  seen  as  high 
as  a  hundred  and  fifty  miles,  and  continue  to  be  visible 
till  within  thirty  miles  of  the  earth.  Their  visible  paths 


Fig.  340. 

vary  from  ten  to  a  hundred  miles  in  length,  though  they 
are  occasionally  two  hundred  or  three  hundred  miles  long. 
Their  average  velocity,  relatively  to  the  earth's  surface,  varies 
from  ten  to  forty-five  miles  a  second. 

The  average  number  of  shooting-stars  visible  to  the 
naked  eye  at  any  one  place  is  estimated  at  about  a  thou- 
sand an  hour ;  and  the  average  number  large  enough  to 
be  visible  to  the  naked  eye,  that  traverse  the  atmosphere 
daily,  is  estimated  at  over  eight  millions.  The  number  of 
telescopic  shooting-stars  would  of  course  be  much  greater. 

Occasionally,  shooting-stars  leave  behind  them  a  trail  of 


3O2  ASTRONOMY. 

light  which  lasts  for  several  seconds.  These  trails  are  some- 
times straight,  as  shown  in  Fig.  344,  and  sometimes  curved, 
as  in  Figs.  345  and  346.  They  often  disappear  like  trails 
of  smoke,  as  shown  in  Fig.  347. 


Fig.  341- 
Shooting-stars  are  seen  to  move  in  all  directions  through 

the  heavens.  Their  apparent  paths  are,  however,  generally 
inclined  downward,  though  sometimes  upward ;  and  after 
midnight  they  come  in  the  greatest  numbers  from  that 
quarter  of  the  heavens  toward  which  the  earth  is  moving 
in  its  journey  around  the  sun. 


ASTRONOMY.  303 

306.  Meteors.  —  Occasionally  these  bodies  are  brilliant 
enough  to  illuminate  the  whole  heavens.  They  are  then 
called  meteors,  although  this  term  is  equally  applicable 


Fig.  342. 

to  ordinary  shooting-stars.  Such  a  meteor  is  shown  in 
Fig.  348. 

Sometimes  these  brilliant  meteors  are  seen  to  explode, 
as  shown  in  Fig.  349  ;  and  the  explosion  is  accompanied 
with  a  loud  detonation,  like  the  discharge  of  cannon. 

Ordinary   shooting-stars   are   not    accompanied    by   any 


3O4  ASTRONOMY. 

audible  sound,  though  they  are  sometimes  seen  to  break 
in  pieces.  Meteors  which  explode  with  an  audible  sound 
are  called  detonating  meteors. 


Fig.  343- 

307.  Aerolites.  —  There  is  no  certain  evidence  that  any 
deposit  from  ordinary  shooting-stars  ever  reaches  the  sur- 


Fig.  344- 

face  of  the  earth ;  though  a  peculiar  dust  has  been  found 
in  certain  localities,  which  has  been  supposed  to  be  of 
meteoric  origin,  and  which  has  been  called  meteoric  dust. 


ASTRONOMY. 


305 


But  solid  bodies  occasionally  descend  to  the  earth  from 
beyond  our  atmosphere.  These  generally  penetrate  a  foot 
or  more  into  the  earth,  and;  if  picked  up  soon  after  their 
fall,  are  found  to  be  warm,  and  sometimes  even  hot.  These 


Fig-  345- 

bodies  are  called  aerolites.  When  they  have  a  stony  appear- 
ance, and  contain  but  little  iron,  they  are  called  meteoric 
stones;  when  they  have  a  metallic  appearance,  and  are 
composed  largely  of  iron,  they  are  called  meteoric  iron. 

There  are  eighteen  well-authenticated  cases  in  which  aero- 
lites have  fallen   in   the    United    States    during  the  last  sixty 


Fig.  346. 

years,  and  their  aggregate  weight  is  twelve  hundred  and  fifty 
pounds.  The  entire  number  of  known  aerolites  the  date  of 
whose  fall  is  well  determined  is  two  hundred  and  sixty-one. 
There  are  also  on  record  seventy-four  cases  of  which  the  date 


ASTRONOMY. 


is  more  or  less  uncertain.  There  have  also  been  found  eighty- 
six  masses,  which,  from  their  peculiar  composition,  are  believed 
to  be  aerolites,  though  their  fall  was  not  seen.  The  weight 


Fig.  347- 

of  these  masses  varies  from  a  few  pounds  to  several  tons. 
The  entire  number  of  aerolites  of  which  we  have,  any  knowl- 
edge is  therefore  about  four  hundred  and  twenty. 


Fig:348. 

Aerolites  are  composed  of  the  same  elementary  substances 
as  occur  in  terrestrial  minerals,  not  a  single  new  element 
having  been  found  in  their  analysis.  Of  the  sixty  or  more 


ASTRONOMY. 


307 


elements  now  recognized  by  chemists,  about  twenty  have  been 
found  in  aerolites. 

While  aerolites  contain  no  new  elements,  their'  appearance 
is  quite  peculiar;  and  the   compounds  found  in   them  are  so 


F'g-  349- 

peculiar  as  to  enable  us  by  chemical  analysis   to  distinguish 
an  aerolite  from  any  terrestrial  substance. 

Iron  ores  are  very  abundant  in  nature,  but  iron  in  the 
metallic  state  is  exceedingly  rare.  Now,  aerolites  invariably 
contain  metallic  iron,  sometimes  from  ninety  to  ninety-six  per 
cent.  This  iron  is  malleable,  and  may  be  readily  worked  into 


Fig.  350. 

cutting  instruments.  It  always  contains  eight  or  ten  per  cent 
of  nickel,  together  with  small  quantities  of  cobalt,  copper,  tin, 
and  chromium.  This  composition  has  never  been  found  in 
any  terrestrial  mineral.  Aerolites  also  contain,  usually  in 
small  amount,  a  compound  of  iron,  nickel,  and  phosphorus, 
which  has  never  been  found  elsewhere, 


308 


ASTRONOMY. 


Meteorites  often  present  the  appearance  of  having  been 
fused  on  the  surface  to  a  slight  depth,  and  meteoric  iron  is 
found  to  have  a  peculiar  crystalline  structure.  The  external 
appearance  of  a  piece  of  meteoric  iron  found  near  Lock- 
port,  N.Y.,  is  shown  in  Fig.  350.  Fig.  351  shows  the  peculiar 
internal  structure  of  meteoric  iron. 

308.  Meteoroids.  —  Astronomers  now  universally  hold 
that  shooting-stars,  meteors,  and  aerolites  are  all  minute 
bodies,  revolving,  like  the  comets,  about  the  sun.  "They 


g-  351 


are  moving  in  every  possible  direction  through  the  celestial 
spaces.  They  may  not  average  more  than  one  in  a  million 
of  cubic  miles,  and  yet  their  total  number  exceeds  all  calcu- 
lation. Of  the  nature  of  the  minuter  bodies  of  this  class 
nothing  is  certainly  known.  The  earth  is  continually  en- 
countering them  in  its  journey  around  the  sun.  They  are 
burned  by  passing  through  the  upper  regions  of  our  atmos- 
phere, and  the  shooting-star  is  simply  the  light  of  that 
burning.  These  bodies,  which  are  invisible  till  they  plunge 
into  the  earth's  atmosphere,  are  called  ineteoroids. 

309.   Origin  of  tJie  Light  of  Meteors.  —  When    one    of 


ASTRONOMY.  309 

these  meteoroids  enters  our  atmosphere,  the  resistance  of 
the  air  arrests  its  motion  to  some  extent,  and  so  converts 
a  portion  of  its  energy  of  motion  into  that  of  heat.  The 
heat  thus  developed  is  sufficient  to  raise  the  meteoroid  and 
the  air  around  it  to  incandescence,  and  in  most  cases 
either  to  cause  the  meteoroid  to  burn  up,  or  to  dissipate  it 
as  vapor.  The  luminous  vapor  thus  formed  constitutes  the 
luminous  train  which  occasionally  accompanies  a  meteor, 
and  often  disappears  as  a  puff  of  smoke.  When  a  meteo- 
roid is  large  enough  and  refractory  enough  to  resist  the 
heat  to  which  it  is  exposed,  its  motion  is  sufficiently  arrested, 
on  entering  the  lower  layers  of  our  atmosphere,  to  cause 
it  to  fall  to  the  earth.  We  then  have  an  aerolite.  A 
brilliant  meteor  differs  from  a  shooting-star  simply  in  mag- 
nitude. 

310.  The  Intensity  of  the  Heat  to  which  a  Meteoroid  is 
Exposed.  —  It  has  been  ascertained  by  experiment  that  a  body 
moving  through  the  atmosphere  at  the  rate  of  a  hundred  and 
twenty-five  feet  a  second  raises  the  temperature  of  the  air 
immediately  in  front  of  it  one  degree,  and  that  the  temperature 
increases  as  the  square  of  the  velocity  of  the  moving  body  ; 
that  is  to  say,  that,  with  a  velocity  of  two  hundred  and  fifty 
feet,  the  temperature  in  front  of  the  body  would  be  raised 
four  degrees ;  with  a  velocity  of  five  hundred  feet,  sixteen 
degrees ;  and  so  on.  To  find,  therefore,  the  temperature  to 
which  a  meteoroid  would  be  exposed  in  passing  through  our 
atmosphere,  we  have  merely  to  divide  its  velocity  in  feet  per 
second  by  a  hundred  and  twenty-five,  and  square  the  quotient. 
With  a  velocity  of  forty-four  miles  a  second  in  our  atmosphere, 
a  meteoroid  would  therefore  be  exposed  to  a  temperature  of 
between  three  and  four  million  degrees.  The  air  acts  upon 
the  body  as  if  it  were  raised  to  this  intense  heat.  At  such  a 
temperature  small  masses  of  the  most  refractory  or  incom- 
bustible substances  known  to  us  would  flash  into  vapor  with 
the  evolution  of  intense  light  and  heat. 

If   one  of  these  meteoric  bodies  is  large  enough  to  pass 


3IO  ASTRONOMY. 

through  the  atmosphere  and  reach  the  earth,  without  being 
volatilized  by  the  heat,  we  have  an  aerolite.  As  it  is  only  a 
few  seconds  in  making  the  passage,  the  heat  has  not  time  to 
penetrate  far  into  its  interior,  but  is  expended  in  melting  and 
vaporizing  the  outer  portions.  The  resistance  of  the  denser 
strata  of  the  atmosphere  to  the  motion  of  the  aerolite  some- 
times becomes  so  enormous  that  the  body  is  suddenly  rent  to 
pieces  with  a  loud  detonation.  It  seems  like  an  explosion  pro- 
duced by  some  disruptive  action  within  the  mass;  but  there 
can  be  little  doubt  that  it  is  due  to  the  velocity  —  perhaps  ten, 
twenty,  or  thirty  miles  a  second  —  with  which  the  body  strikes 
the  air. 

If,  on  the  other  hand,  the  meteoroid  is  so  small  as  to  be 
burned  up  or  volatilized  in  the  upper  regions  of  the  atmos- 
phere, we  have  a  common  shooting-star,  or  a  meteor  of  greater 
or  less  brilliancy. 

311.  Meteoric  Showers.  —  On  ordinary  nights  only  four 
or  five  shooting-stars  are  seen  in  an  hour,  and  these  move 
in  every  direction.  Their  orbits  lie  in  all  possible  positions, 
and  are  seemingly  scattered  at  random.  Such  meteors  are 
called  sporadic  meteors.  On  occasional  nights,  shooting- 
stars  are  more  numerous,  and  all  move  in  a  common  direc- 
tion. Such  a  display  is  called  a  meteoric  shower.  These 
showers  differ  greatly  in  brilliancy;  but  during  any  one 
shower  the  meteors  all  appear  to  radiate  from  some  one 
point  in  the  heavens.  If  we  mark  on  a  celestial  globe  the 
apparent  paths  of  the  meteors  which  fall  during  a  shower, 
or  if  we  trace  them  back  on  the  celestial  sphere,  we  shall 
find  that  they  all  meet  in  the  same  point,  as  shown  in  Fig. 
352.  This  point  is  called  the  radiant  point.  It  always 
appears  in  the  same  position,  wherever  the  observer  is  situ- 
ated, and  does  not  partake  of  the  diurnal  motion  of  the 
earth.  As  the  stars  move  towards  the  west,  the  radiant 
point  moves  with  them.  The  point  in  question  is  purely 
an  effect  of  perspective,  being  the  "vanishing  point"  of 
the  parallel  lines  in  which  the  meteors  are  actually  moving. 


ASTRONOMY.  31 1 

These  lines  are  seen,  not  in  their  real  direction  in  space, 
but  as  projected  on  the  celestial  sphere.  If  we  look  up- 
wards, and  watch  snow  falling  through  a  calm  atmosphere, 
the  flakes  which  fall  directly  towards  us  do  not  seem  to 
move  at  all,  while  the  surrounding  flakes  seem  to  diverge 
from  them  on  all  sides.  So,  in  a  meteoric  shower,  a 
meteor  coming  directly  towards  the  observer  does  not  seem 


Fig.  352. 

to   move   at  all,  and  marks  the  point  from  which  all  the 
others  seem  to  radiate. 

312.  The  August  Meteors.  —  A  meteoric  shower  of  no 
great  brilliancy  occurs  annually  about  the  loth  of  August. 
The  radiant  point  of  this  shower  is  in  the  constellation  Per- 
seus, and  hence  these  meteors  are  often  called  the  Perseids. 
The  orbit  of  these  meteoroids  has  been  pretty  accurately 
determined,  and  is  shown  in  Fig.  353. 


312 


ASTRONOMY. 


It  will  be  seen   that  the  perihelion  point  of  this  orbit  is 
at  about  the  distance  of  the  earth  from  the  sun  ;   so  that 

the  earth  encounters  the  mete- 
ors once  a  year,  and  this  takes 
place  in  the  month  of  August. 
The  orbit  is  a  very  eccentric 
ellipse,  reaching  far  beyond 
Neptune.  As  the  meteoric  dis- 
play is  about  equally  brilliant 
every  year,  it  seems  probable 
that  the  meteoroids  form  a 
stream  quite  uniformly  dis- 
^  tributed  throughout  the  whole 
orbit.  It  probably  takes  one 
of  the  meteoroids  about  a  hun- 
dred and  twenty-four  years  to 
pass  around  this  orbit. 

313.  The  November  Meteors. 
—  A  somewhat  brilliant  mete- 
oric shower  also  occurs  annu- 
ally, about  the  i3th  of  Novem- 
ber. The  radiant  point  of 
these  meteors  is  in  the  con- 
stellation Leo,  and  hence  they 
are  often  called  the  Leonids. 
Their  orbit  has  been  deter- 
mined with  great  accuracy,  and 
is  shown  in  Fig.  354.  While 
the  November  meteors  are  not 
usually  very  numerous  or  bright, 
a  remarkably  brilliant  display 
of  them  has  been  seen  once  in 
about  thirty-three  or  thirty-four  years  :  hence  we  infer,  that, 
while  there  are  some  meteoroids  scattered  throughout  the 
whole  extent  of  the  orbit,  the  great  majority  are  massed  in 


Fig.  353- 


ASTRONOMY. 


313 


a  group  which  traverses  the  orbit  in  a  little  over  thirty  - 
three  years.  A  conjectural  form  of  this  condensed  group  is 
shown  in  Fig.  355.  The  group  is  so  large  that  it  takes  it 
two  or  three  years  to  pass  the  perihelion  point :  hence  there 
may  be  a  brilliant  meteoric  display  two  or  three  years  in 
succession. 

The  last  brilliant  display  of  these  meteors  was  in  the 
years  1866  and  1867. 
The  display  was  visible 
in  this  country  only  a 
short  time  before  sun- 
rise, and  therefore  did 
not  attract  general  atten- 
tion. The  display  of 
1833  was  remarkably 
brilliant  in  this  country, 
and  caused  great  con- 
sternation among  the 
ignorant  and  supersti- 
tious. 

314.  Connect  ion  between 
Meteors  and  Comets.  —  It 
has  been  found  that  a 
comet  which  appeared  in 
1866,  and  which  is  desig- 
nated as  1866,  I.,  has 
exactly  the  same  orbit 
and  period  as  the  No-  Fig.  354. 

vember  meteors,  and  that  another  comet,  known  as  the  1862, 
III.,  has  the  same  orbit  as  the  August  meteors.  It  has  also 
been  ascertained  that  a  third  comet,  1861,  I.,  has  the  same 
orbit  as  a  stream  of  meteors  which  the  earth  encounters  in 
April.  Furthermore,  it  was  found,  in  1872,  that  there  was  a 
small  stream  of  meteors  following  in  the  train  of  the  lost 
comet  of  Biela.  These  various  orbits  of  comets  and  meteoric 
streams  are  shown  in  Fig.  356.  The  coincidence  of  the  orbits 


314  ASTRONOMY. 

of  comets  and  of  meteoric  streams  indicates  that  these  two 
classes  of  bodies  are  very  closely  related.  They  undoubtedly 
have  a  common  origin.  The  fact  that  there  is  a  stream  of 
meteors  in  the  train  of  Biela's  comet  has  led  to  the  sup- 
position that  comets  may  become  gradually  disintegrated  into 
meteoroids, 

PHYSICAL  AND  CHEMICAL  CONSTITUTION  OF  COMETS. 

315.  Physical  Constitution  of  Telescopic  Comets.  —  We  have 
no  certain  knowledge  of  the  physical  constitution  of  telescopic 

comets.  They  are  usually  tens 
of  thousands  of  miles  in  diame- 
ter, and  yet  of  such  tenuity  that 
the  smallest  stars  can  readily  be 
seen  through  them.  It  would 
seem  that  they  must  shine  in 
part  by  reflected  light ;  yet  the 
spectroscope  shows  that  their 
spectrum  is  composed  of  bright 
bands,  which  would  indicate  that 
they  are  composed,  in  part  at 
least,  of  incandescent  gases.  It 
is,  however,  difficult  to  conceive 
how  these  gases  become  suf- 
ficiently heated  to  be  luminous ; 
and  at  the  same  time  such 
gases  would  reflect  no  sunlight. 

It  seems  probable  that  these 
comets  are  really  made  up  of  a 
combination  of  small,  solid  par- 
ticles in  the  form  of  minute 
meteoroids,  and  of  gases  which 
are,  perhaps,  rendered  luminous 
by  electric  discharges  of  slight 
Flg-  355-  intensity. 

316.  Physical  Constitution   of  Large  Comets.  —  In  the  case 
of   large  comets  the  nucleus  is  either  a  dense  mass  of  solid 
matter  several  hundred  miles  in  diameter,  or  a  dense  group 
of   meteoroids.     Professor  Peirce    estimated   that  the  density 


ASTRONOMY. 


315 


of  the  nucleus  is  at  least  equal  to  that  of  iron.  As  such  a 
comet  approaches  the  sun,  the  nucleus  is,  to  a  slight  extent, 
vaporized,  and  out  of  this  vapor  is  formed  the  coma  and  the 
tail. 

That  some  evaporating  process  is  going  on  from  the  nucleus 
of  the  comet  is  proved  by  the  movements  of  the  tail.  It  is 
evident  that  the  tail  cannot  be  an  appendage  carried  along  with 
the  comet,  as  it  seems  to  be.  It  is  impossible  that  there 
should  be  any  cohesion  in  matter  of  such  tenuity  that  the 
smallest  stars  could  be  seen  through  a  million  of  miles  of  it, 
and  which  is,  moreover,  continually  changing  its  form.  Then, 


Fig.  356. 

again,  as  a  comet  is  passing  its  perihelion,  the  tail  appears  to 
be  whirled  from  one  side  of  the  sun  to  another  with  a  rapidity 
which  would  tear  it  to  pieces  if  the  movement  were  real.  The 
tail  seems  to  be,  not  something  attached  to  the  comet,  and 
carried  along  with  it,  but  a  stream  of  vapor  issuing  from  it, 
like  smoke  from  a  chimney.  The  matter  of  which  it  is  com- 
posed is  continually  streaming  outwards,  and  continually  being 
replaced  by  fresh  vapor  from  the  nucleus. 

The  vapor,  as  it  emanates  from  the  nucleus,  is  repelled  by 
the  sun  with  a  force  often  two  or  three  times  as  great  as  the 
ordinary  solar  attraction.  The  most  probable  explanation  of 
this  phenomenon  is,  that  it  is  a  case  of  electrical  repulsion,  the 
sun  and  the  particles  of  the  cometary  mist  being  similarly 


ASTRONOMY. 


electrified.  With  reference  to  this  electrical  theory  of  the 
formation  of  comets'  tails,  Professor  Peirce  makes  the  follow- 
ing observation :  "In  its  approach  to  the  sun,  the  surface  of 
the  nucleus  is  rapidly  heated :  it  is  melted  and  vaporized,  and 
subjected  to  frequent  explosions.  The  vapor  rises  in  its  atmos- 
phere with  a  well-defined  upper  surface,  which  is  known  to 
observers  as  an  envelop.  .  .  .  The  electrification  of  the  come- 
tary  mist  is  analogous  to  that  of  our  own  thunder-clouds.  Any 
portion  of  the  coma  which  has  received  the  opposite  kind  of 
electricity  to  the  sun  and  to  the  repelled  tail  will  be  attracted. 
This  gives  a  simple  explanation  of  the  negative  tails  which  have 
been  sometimes  seen  directed  towards  the  sun.  In  cases  of 
violent  explosion,  the  whole  nucleus  might  be  broken  to  pieces, 

and  the  coma  dashed 
around  so  as  to  give 
varieties  of  tail,  and  even 
multiple  tails.  There 
seems,  indeed,  to  be  no 
observed  phenomenon  of 
the  tail  or  the  coma 
which  is  not  consistent 
with  a  reasonable  modifi- 
cation of  the  theory.*' 
Professor  Peirce  regard- 
ed comets  simply  as  the 
largest  of  the  meteoroids. 
They  appear  to  shine  partly  by  reflected  sunlight,  and  partly 
by  their  own  proper  light,  which  seems  to  be  that  of  vapor 
rendered  luminous  by  an  electric  discharge  of  slight  intensity. 

317.  Collision  of  a  Comet  and  tJic  Earth.  —  It  sometimes 
happens  that  the  orbit  of  a  comet  intersects  that  of  the  earth, 
as  is  shown  in  Fig.  357,  which  shows  a  portion  of  the  orbit 
of  Biela's  comet,  with  the  positions  of  the  comet  and  of  the 
earth  in  1832.  Of  course,  were  a  comet  and  the  earth  both  to 
reach  the  intersection  of  their  orbits  at  the  same  time,  a  col- 
lision of  the  two  bodies  would  be  inevitable.  With  reference 
to  the  probable  effect  of  such  a  collision,  Professor  Newcomb 
remarks,  — 

"The    question    is    frequently    asked,    What  would   be    the 


ASTRONOMY.  317 

effect  if  a  comet  should  strike  the  earth  ?  This  would  depend 
upon  what  sort  of  a  comet  it  was,  and  what  part  of  the  comet 
came  in  contact  with  our  planet.  The  latter  might  pass  through 
the  tail  of  the  largest  comet  without  the  slightest  effect  being 
produced ;  the  tail  being  so  thin  and  airy  that  a  million  miles 
thickness  of  it  looks  only  like  gauze  in  the  sunlight.  It  is  not 
at  all  unlikely  that  such  a  thing  may  have  happened  without 
ever  being  noticed.  A  passage  through  a  telescopic  comet 
would  be  accompanied  by  a  brilliant  meteoric  shower,  probably 
a  far  more  brilliant  one  than  has  ever  been  recorded.  No 
more  serious  danger  would  be  encountered  than  that  arising 
from  a  possible  fall  of  meteorites;  but  a  collision  between 
the  nucleus  of  a  large  comet  and  the  earth  might  be  a  serious 


Fig.  358. 

matter.  If,  as  Professor  Peirce  supposes,  the  nucleus  is  a  solid 
body  of  metallic  density,  many  miles  in  diameter,  the  effect 
where  the  comet  struck  would  be  terrific  beyond  conception. 
At  the  first  contact  in  the  upper  regions  of  the  atmosphere,  the 
whole  heavens  would  be  illuminated  with  a  resplendence 
beyond  that  of  a  thousand  suns,  the  sky  radiating  a  light  which 
would  blind  every  eye  that  beheld  it,  and  a  heat  which  would 
melt  the  hardest  rocks.  A  few  seconds  of  this,  while  the 
huge  body  was  passing  through  the  atmosphere,  and  the  col- 
lision at  the  earth's  surface  would  in  an  instant  reduce  every 
thing  there  existing  to  fiery  vapor,  and  bury  it  miles  deep  in 
the  solid  earth.  Happily,  the  chances  of  such  a  calamity  are 
so  minute  that  they  need  not  cause  the  slightest  uneasiness. 
There  is  hardly  a  possible  form  of  death  which  is  not  a  thou- 
sand times  more  probable  than  this.  So  small  is  the  earth  in 


ASTRONOMY. 

comparison  with  the  celestial  spaces,  that  if  one  should  shut 
his  eyes,  and  fire  a  gun  at  random  in  the  air,  the  chance  of 
bringing  down  a  bird  would  be  better  than  that  of  a  comet  of 
any  kind  striking  the  earth." 

318.  The  Chemical  Constitution  of  Comets.  —  Fig.  358  shows 
the  bands  of  the  spectrum  of  a  telescopic  comet  of  1873,  as 
seen  by  two  different  observers.  Fig.  359  shows  the  spectrum 
of  the  coma  and  tail  of  the  comet  of  1874;  and  the  spectrum 
of  the  bright  comet  of  1881  showed  the  same  three  bands  for 


359- 

the  coma  and  tail.  Now,  these  three  bands  are  those  of  cer- 
tain hydrocarbon  vapors :  hence  it  would  seem  that  the  coma 
and  tails  of  comets  are  composed  chiefly  of  such  vapors  (315). 

II.    THE  ZODIACAL   LIGHT. 

319.  The  General  Appearance  of  the  Zodiacal  Light. — 
The  phenomenon  known  as  the  zodiacal  light  consists  of  a 
very  faint  luminosity,  which  may  be  seen  rising  from  the 
western  horizon  after  twilight  on  any  clear  winter  or  spring 
evening,  also  from  the  eastern  horizon  just  before  daybreak 
in  the  summer  or  autumn.  It  extends  out  on  each  side 
of  the  sun,  and  lies  nearly  in  the  plane  of  the  ecliptic.  It 
grows  fainter  the  farther  it  is  from  the  sun,  and  can  gener- 
ally be  traced  to  about  ninety  degrees  from  that  luminary, 


ASTRONOMY. 


319 


Fig.  360. 


320 


ASTRONOMY. 


when  it  gradually  fades  away.  In  a  very  clear,  tropical 
atmosphere,  it  has  been  traced  all  the  way  across  the 
heavens  from  east  to  west,  thus  forming  a  complete  ring. 
The  general  appearance  of  this  column  of  light,  as  seen 
in  the  morning,  in  the  latitude  of  Europe,  is  shown  in 
Fig.  360. 

Taking  all  these  appearances  together,  they  indicate  that 
it  is  due  to  a  lens-shaped  appendage  surrounding  the  sun, 

and  extending  a  little 
beyond  the  earth's 
orbit.  It  lies  nearly 
in  the  plane  of  the 
ecliptic  ;  but  its  exact 
position  is  not  easily 


determined. 
361   shows  the 
eral  form  and 
lion     of    this 
appendage,    as    seen 
in  the  west. 


Fig. 
gen- 
posi- 
solar 


320.  The  Visibility 
of  the  Zodiacal  Light. 
—  The  reason  why  the 
zodiacal  light  is  more 
favorably  seen  in  the 
evening  during  the 
winterand  spring  than 

in  the  summer  and  fall  is  evident  from  Fig.  362,  which  shows 
the  position  of  the  ecliptic  and  the  zodiacal  light  with  reference 
to  the  western  horizon  at  the  time  of  sunset  in  March  and  in 
September.  It  will  be  seen  that  in  September  the  axis  of  the 
light  forms  a  small  angle  with  the  horizon,  so  that  the  phenom- 
enon is  visible  only  a  short  time  after  sunset  and  low  down 
where  it  is  difficult  to  distinguish  it  from  the  glimmer  of  the  twi- 
light; while  in  March,  its  axis  being  nearly  perpendicular  to  the 
horizon,  the  light  may  be  observed  for  some  hours  after  sunset 


ASTRONOMY. 


321 


and  well  up  in  the  sky.      Fig.  363  gives  the   position  of   the 

ecliptic  and  of  the  zodiacal  light  with  reference  to  the  eastern 

horizon  at  the  time  of 

sunrise,  and  shows  why 

the     zodiacal    light    is 

seen   to    better  advan- 

tage   in     the    morning 

during  the  summer  and 

fall    than     during    the 

winter  and  spring.      It 

will   be   observed    that 

here  the  angle  made  by 

the   axis    of    the    light' 

with  the  horizon  is  small 

in    March,    while    it   is 

large  in  September;  the 

conditions  represented 

in  the  preceding  figure 

being  thus  reversed.  Fig.  362. 

321.  Nature  of  the  Zodiacal  Light.  —  Various  attempts  have 

been  made  to  explain 
the  phenomena  of  the 
zodiacal  light  ;  but  the 
most  probable  theory 
is,  that  it  is  due  to 
an  immense  number  of 
meteors  which  are  re- 
volving around  the  sun, 
and  which  lie  mostly 
within  the  earth's  orbit. 
Each  of  these  meteors 
reflects  a  sensible  por- 
tion of  sunlight,  but  is 
far  too  small  to  be  sep- 
arately visible.  All  of 
these  meteors  together 
would,  b  their  com- 


is- 363- 


bined  reflection,  produce  a  kind  of  pale,  diffused  light. 


III. 

THE   STELLAR   UNIVERSE. 


I.     GENERAL   ASPECT   OF   THE   HEAVENS. 

322.  The  Magnitude  of  the  Stars.  —  The  stars  that  are 
visible  to  the  naked  eye  are  divided  into  six  classes,  accord- 
ing to  their  brightness.  The  brightest  stars  are  called  stars 
of  the  first  magnitude ;  the  next  brightest,  those  of  the 
second  magnitude  ;  and  so  on  to  the  sixth  magnitude.  The 
last  magnitude  includes  the  faintest  stars  that  are  visible  to 
the  naked  eye  on  the  most  favorable  night.  Stars  which 
are  fainter  than  those  of  the  sixth  magnitude  can  be  seen 
only  with  the  telescope,  and  are  called  telescopic  stars. 
Telescopic  stars  are  also  divided  into  magnitudes  ;  the  divis- 
ion extending  to  the  sixteenth  magnitude,  or  the  faintest 
stars  that  can  be  seen  with  the  most  powerful  telescopes. 

The  classification  of  stars  according  to  magnitudes  has 
reference  only  to  their  brightness,  and  not  at  all  to  their 
actual  size.  A  sixth  magnitude  star  may  actually  be  larger 
than  a  first  magnitude  star ;  its  want  of  brilliancy  being  due 
to  its  greater  distance,  or  to  its  inferior  luminosity,  or  to 
both  of  these  causes. 

None  of  the  stars  present  any  sensible  disk,  even  in  the 
most  powerful  telescope  :  they  all  appear  as  mere  points  of 
light.  The  larger  the  telescope,  the  greater  is  its  power 
of  revealing  faint  stars ;  not  because  it  makes  these  stars 
appear  larger,  but  because  of  its  greater  light-gathering 
322 


ASTRONOMY.  323 

power.  This  power  increases  with  the  size  of  the  object- 
glass  of  the  telescope,  which  plays  the  part  of  a  gigantic 
pupil  of  the  eye. 

The  classification  of  the  stars  into  magnitudes  is  not 
made  in  accordance  with  any  very  accurate  estimate  of 
their  brightness.  The  stars  which  are  classed  together  in 
the  same  magnitude  are  far  from  being  equally  bright. 

The  stars  of  each  lower  magnitude  are  about  two-fifths  as 
bright  as  those  of  the  magnitude  above.  The  ratio  of  diminu- 
tion is  about  a  third  from  the  higher  magnitude  down  to  the 
fifth.  Were  the  ratio  two-fifths  exact,  it  would  take  about 

2\  stars  of  the  2cl     magnitude  to  make  one  of  the  1st. 

6  stars  of  the  jcl     magnitude  to  make  one  of  the  ist. 

16  stars  of  the  4th    magnitude  to  make  one  of  the  ist. 

40  stars  of  the  5th    magnitude  to  make  one  of  the  ist. 

100  stars  of  the  6th    magnitude  to  make  one  of  the  ist. 

10,000  stars  of  the  nth  magnitude  to  make  one  of  the  ist. 

1,000,000  stars  of  the  i6th  magnitude  to  make  one  of  the  ist. 

323.  The  Number  of  the  Stars.  —  The  total  number 
of  stars  in  the  celestial  sphere  visible  to  the  average  naked 
eye  is  estimated,  in  round  numbers,  at  five 
thousand  ;  but  the  number  varies  much  with 
the  perfection  and  the  training  of  the  eye 
and  with  the  atmospheric  conditions.  For 
every  star  visible  to  the  naked  eye,  there  are 
thousands  too  minute  to  be  seen  without  Fig- 
telescopic  aid.  Fig.  364  shows  a  portion  of  the  constella- 
tion of  the  Twins  as  seen  with  the  naked  eye ;  and  Fig.  365 
shows  the  same  region  as  seen  in  a  powerful  telescope. 

Struve  has  estimated  that  the  total  number  of  stars  visible 
with  Herschel's  twenty-foot  telescope  was  about  twenty 
million.  The  number  that  can  be  seen  with  the  great  tele- 
scopes of  modern  times  has  not  been  carefully  estimated, 
but  is  probably  somewhere  between  thirty  million  and  fifty 
million. 


3^4 


ASTRONOMY. 


The  number  of  stars  between  the  north  pole  and  the  circle 
thirty-five  degrees  south  of  the  equator  is  about  as  follows :  — 

Of  the  ist  magnitude  about 14  stars. 

Of  the  2d    magnitude  about 48  stars. 

Of  the  3d    magnitude  about 152  stars. 

Of  the  4th  magnitude  about 313  stars. 

Of  the  5th  magnitude  about 854  stars. 

Of  the  6th  magnitude  about 2010  stars. 

Total  visible  to  naked  eye 3391  stars. 


Fig.  365- 

The  number  of  stars  of  the  several  magnitudes  is  approxi- 
mately in  inverse  proportion  to  that  of  their  brightness,  the 
ratio  being  a  little  greater  in  the  higher  magnitudes,  and  proba- 
bly a  little  less  in  the  lower  ones. 


ASTRONOMY.  325 

324.  The  Division    of  the  Stars   into   Constellations. — 
A  glance  at  the  heavens  is  sufficient  to  show  that  the  stars 
are  not  distributed  uniformly  over  the  sky.     The  larger  ones 
especially  are  collected  into  more  or  less  irregular  groups. 
The  larger  groups  are  called  constellations.     At  a  very  early 
period  a  mythological  figure  was  allotted  to  each  constella- 
tion ;   and  these  figures  were  drawn  in  such   a  way  as  to 
include    the    principal    stars    of    each    constellation.     The 
heavens  thus   became   covered,   as   it  were,  with  immense 
hieroglyphics. 

There  is  no  historic  record  of  the  time  when  these  figures 
were  formed,  or  of  the  principle  in  accordance  with  which  they 
were  constructed.  It  is  probable  that  the  imagination  of  the 
earlier  peoples  may,  in  many  instances,  have  discovered  some 
fanciful  resemblance  in  the  configuration  of  the  stars  to  the 
forms  depicted.  The  names  are  still  retained,  although  the 
figures  no  longer  serve  any  astronomical  purpose.  The  con- 
stellation Hercules,  for  instance,  no  longer  represents  the  figure 
of  a  man  among  the  stars,  but  a  certain  portion  of  the  heavens 
within  which  the  ancients  placed  that  figure.  In  star-maps 
intended  for  school  and  popular  use  it  is  still  customary  to 
give  these  figures ;  but  they  are  not  generally  found  on  maps 
designed  for  astronomers. 

325.  The  Naming  of  the  Stars.  —  The  brighter  stars  have 
all  proper  names,   as   Strius,   Procyon,   Arctiirus,   Capella, 
Aldebaran,  etc.     This  method  of  designating  the  stars  was 
adopted  by  the  Arabs.     Most  of  these  names  have  dropped 
entirely  out  of  astronomical  use,  though  many  are  popularly 
retained.     The  brighter  stars  are  now  generally  designated 
by  the  letters  of  the  Greek  alphabet.  —  alplia.  beta,  gamma, 
etc.,  —  to  which  is  appended  the  genitive  of  the  name  of 
the  constellation,  the  first  letter  of  the  alphabet  being  used 
for  the  brightest  star,  the  second  for  the  next  brightest,  and 
so   on.     Thus  Aldebaran   would    be    designated    as   Alpha 
Tauri.     In  speaking  of  the  stars  of  any  one  constellation, 


326 


ASTRONOMY. 


we  simply  designate  them  by  the  letters  of  the  Greek  alpha- 
bet, without  the  addition  of  the  name  of  the  constellation, 
which  answers  to  a  person's  surname,  while  the  Greek  letter 
answers  to  his  Christian  name.  The  names  of  the  seven 
stars  of  the  "  Dipper  "  are  given  in  Fig.  366.  When  the 
letters  of  the  Greek  alphabet  are  exhausted,  those  of  the 
Roman  alphabet  are  employed.  The  fainter  stars  in  a 
constellation  are  usually  designated  by  some  system  of 
numbers. 

326.   The  Milky -Way,  or  Galaxy.  — lite  Milky-Way  is 


Fig.  366. 

a  faint  luminous  band,  of  irregular  outline,  which  surrounds 
the  heavens  with  a  great  circle,  as  shown  in  Fig.  367. 
Through  a  considerable  portion  of  its  course  it  is  divided 
into  two  branches,  and  there  are  various  vacant  spaces  at 
different  points  in  this  band ;  but  at  only  one  point  in  the 
southern  hemisphere  is  it  entirely  interrupted. 

The  telescope  shows  that  the  Galaxy  arises  from  the 
light  of  countless  stars  too  minute  to  be  separately  visible 
with  the  naked  eye.  The  telescopic  stars,  instead  of  being 
uniformly  distributed  over  the  celestial  sphere,  are  mostly 


ASTRONOMY. 


327 


Fig.  367- 


328 


ASTRONOMY. 


condensed  in  the  region  of  the  Galaxy.  They  are  fewest 
in  the  regions  most  distant  from  this  belt,  and  become 
thicker  as  we  approach  it.  The  greater  the  telescopic 
power,  the  more  marked  is  the  condensation.  With  the 
naked  eye  the  condensation  is  hardly  noticeable ;  but  with 

the  aid  of  a  very  small  tele- 
scope, we  see  a  decided  thicken- 
ing of  the  stars  in  and  near  the 
Galaxy,  while  the  most  power- 
ful telescopes  show  that  a  large 
majority  of  the  stars  lie  actually 
in  the  Galaxy.  If  all  the  stars 
visible  with  a  twelve-inch  tele- 
scope were  blotted  out,  we 
should  find  that  the  greater  part 
of  those  remaining  were  in  the 
Galaxy. 

The  increase  in  the  number 
of  the  stars  of  all  magnitudes  as 
we  approach  the  plane  of  the 
Milky- Way  is  shown  in  Fig.  368. 
The  curve  acb  shows  by  its 
height  the  distribution  of  the 
stars  above  the  ninth  magnitude, 
and  the  curve  A  C B  those  of 
all  magnitudes. 

327.  Star  -  Clusters.  —  Besides 
this  gradual  and  regular  con- 
densation towards  the  Galaxy, 
occasional  aggregations  of  stars 

into  clusters  may  be  seen.  Some  of  these  are  visible  to  the 
naked  eye,  sometimes  as  separate  stars,  like  the  "  Seven 
Stars,"  or  Pleiades,  but  more  commonly  as  patches  of  dif- 
fused light,  the  stars  being  too  small  to  be  seen  separately. 
The  number  visible  in  powerful  telescopes  is,  however,  much 


ASTRONOMY. 


329 


Fig.  369. 


330  ASTRONOMY. 

greater.  Sometimes  hundreds  or  even  thousands  of  stars 
are  visible  in  the  field  of  view  at  once,  and  sometimes  the 
number  is  so  great  that  they  cannot  be  counted. 

328.  Nebula.  —  Another  class  of  objects  which  are  found 
in  the  celestial  spaces  are  irregular  masses  of  soft,  cloudy 
light,  known    as    nebula.     Many  objects   which    look    like 
nebulae   in   small  telescopes   are  shown  by  more    powerful 
instruments  to  be  really  star-clusters.     But  many  of  these 
objects  are  not  composed  of  stars  at  all,  being  immense 
masses  of  gaseous  matter. 

The  general  distribution  of  nebulae  is  the  reverse  of  that 
of  the  stars.  Nebulae  are  thickest  where  stars  are  thinnest. 
While  stars  are  most  numerous  in  the  region  of  the  Milky- 
Way,  nebulae  are  most  abundant  about  the  poles  of  the 
Milky- Way.  This  condensation  of  nebulae  about  the  poles 
of  the  Milky-Way  is  shown  in  Figs.  367  and  369,  in  which 
the  points  represent,  not  stars,  but  nebulae. 

II.     THE    STARS. 
THE  CONSTELLATIONS. 

329.  The  Great  Bear.  —  The    Great  Bear,  or  Ursa  Major, 
is  one  of  the  circumpolar  constellations  (4),  and  contains  one 
of   the    most   familiar   asterisms,   or   groups    of   stars,    in    our 
sky;    namely,   the    Great  Dipper,  or  Charleses   Wain.      The 
positions  and  names   of   the  seven  prominent  stars  in  it  are 
shown  in  Fig.  370.     The  two  stars  Alpha  and  Beta  are  called 
the  Pointers.     This  asterism  is  sometimes  called  the  Butchers 
Cleaver.     The  whole   constellation  is  shown  in  Fig.  371.     A 
rather  faint  star  marks  the  nose  of  the  bear,  and  three  equi- 
distant pairs  of  faint  stars  mark  his  feet. 

330.  The  Little  Bear,  Draco,  and  Cassiopeia.  —  These  are 
all  circumpolar  constellations.     The  most  important  star  of  the 
Little  Bear,  or  Ursa  Minor,  is  Polaris,  or  the  Pole  Star.     This 
star  may  be  found  by  drawing  a  line  from  Beta  to  Alpha  of  the 
Dipper,  and  prolonging  it  as  shown  in  Fig.  372.     This  explains 
why  these  stars  are  called  the   Pointers.     The  Pole  Star,  with 


ASTRONOMY.  33! 

the  six  other  chief  stars  of  the  Little  Bear,  form  an  asterism 


Fig.  370. 

called   the   Little  Dipper.     These   six   stars   are   joined   with 
Polaris  by  a  dotted  line  in  Fig.  372. 


Fig.  371- 

The  stars  in  a  serpentine  line  between  the  two  Dippers  are 
the  chief  stars  of  Draco,  or  the  Dragon ;  the  trapezium  mark- 


332  ASTRONOMY. 

ing  its  head.     Fig.  373  shows  the  constellations  of  Ursa  Minor 
and  Draco  as  usually  figured. 


Fig.  372. 

To  find  Cassiopeia,  draw  a  line  from  Delta  of  the   Dipper  to 


ASTRONOMY. 


333 


Fig.  373- 

Polaris,  and   prolong   it   about   an  equal  distance  beyond,  as 


Fig.  374- 

shown  in  Fig.  372.     This  line  will  pass  near  Alpha  of  Cassio- 


334 


ASTRONOMY. 


peia.  The  five  principal  stars  of  this  constellation  form  an 
irregular  IV,  opening  towards  the  pole.  Between  Cassiopeia 
and  Draco  are  five  rather  faint  stars,  which  form  an  irregular 
K.  These  are  the  principal  stars  of  the  constellation  Cepheus. 
These  two  constellations  are  shown  in  Fig.  374. 

331.  The  Lion,  Berenice's  Hair,  and  the  Hunting-Dogs. — 
A  line  drawn  from  Alpha  to  Beta  of  the  Dipper,  and  prolonged 
as  shown  in  Fig.  375,  will  pass  between  the  two  stars  Denebola 
and  Regulus  ot'Leo,  or  the  Lion.  Regulus  forms  a  sickle  with 


Fig.  375- 

several  other  faint  stars,  and  marks  the  heart  of  the  lion. 
Denebola  is  at  the  apex  of  a  right-angled  triangle,  which  it 
forms  with  two  other  stars,  and  marks  the  end  of  the  lion's 
tail.  This  constellation  is  visible  in  the  evening  from  February 
to  July,  and  is  figured  in  Fig.  376. 

In  a  straight  line  between  Denebola  and  Eta,  at  the  end  of 
the  Great  Bear's  tail,  are,  at  about  equal  distances,  the  two 
small  constellations  of  Coma  Berenices,  or  Berenice's  Hair, 
and  Canes  Venatici,  or  the  Hnnting-Dogs.  These  are  shown 
in  Fig.  377.  The  dogs  are  represented  as  pursuing  the  bear, 
urged  on  by  the  huntsman  Bootes. 


ASTRONOMY. 


335 


Fig.  376. 

332.  Bootes,   Hercules,   and   the    Northern    Crown.  —  Arc 


Fig.  377- 


turns,  the  principal  star  of  Bootes,  may  be  found  by  drawing 


336 


ASTRONOMY. 


a  line  from  Zeta  to  Eta  of  the  Dipper,  and  then  prolonging 
it  with  a  slight  bend,  as  shown  in  Fig.  378.  Arcturus  and 
Polaris  form  a  large  isosceles  triangle  with  a  first-magnitude 
star  called  Vega.  This  triangle  encloses  at  one  corner  the 
principal  stars  of  Bootes,  and  the  head  of  the  Dragon  near 
the  opposite  side.  The  side  running  from  Arcturus  to  Vega 
passes  through  Corona  Borealis,  or  the  ATorthern  Crown,  and 


Fig.  378. 

the  body  of  Hercules,  which  is  marked  by  a  quadrilateral  of 
four  stars. 

Bootes,  who  is  often  represented  as  a  husbandman.  Corona 
Borealis,  and  Hercules,  are  delineated  in  Fig.  379.  These 
constellations  are  visible  in  the  evening  from  May  to  September. 

333.  The  Lyre,  the  Swan,  the  Eagle,  and  the  Dolphin.  — 
Altair,  the  principal  star  of  Aquila,  or  the  Eagle,  lies  on  the 
opposite  side  of  the  Milky-Way  from  Vega.  Altair  is  a  first- 


ASTRONOMY.  337 

magnitude  star,  and  has  a  faint  star  on  each  side  of   it,  as 


Fig.  379. 

shown  in   Fig.  380.     Vega,  also  of  the  first  magnitude,  is  the 


338 


ASTRONOMY. 


principal  star  of  Lyra,  or  the  Lyre.  Between  these  two  stars, 
and  a  little  farther  to  the  north,  are  several  stars  arranged  in 
the  form  of  an  immense  cross.  The  bright  star  at  the  head 
of  this  cross  is  called  Deneb.  The  cross  lies  in  the  Milky- Way, 
and  contains  the  chief  stars  of  the  constellation  Cygnus,  or 
the  Swan.  A  little  to  the  north  of  Altai r  are  four  stars  in  the 
form  of  a  diamond.  This  asterism  is  popularly  known  as  Job's 


Fig.  380. 

Coffin.  These  four  stars  are  the  chief  stars  of  Delphinus^  or 
the  Dolphin.  These  four  constellations  are  shown  together  in 
Fig.  381.  The  Swan  is  visible  from  June  to  December,  in  the 
evening. 

334.  Virgo.  —  A  line  drawn  from  Alpha  to  Gamma  of  the 
Dipper,  and  prolonged  with  a  slight  bend  at  Gamma,  will  reach 
to  a  first-magnitude  star  called  Spica  (Fig.  382).  This  is  the 
chief  star  of  the  constellation  Virgo,  or  the  Virgin,  and  forms 
a  large  isosceles  triangle  with  Arcturus  and  Denebola. 


ASTRONOMY.  339 

Virgo  is  represented  in  Fig.  383.     To  the  right  of  this  con- 


Fig.  38i. 
stellation,  as  shown  in  the  figure,  there  are  four  stars  which 


Fig.  382. 
form  a  trapezium,  and  mark  the  constellation  Corvus,  or  the 


34°  ASTRONOMY, 

Crow.  This  bird  is  represented  as  standing  on  the  body  of 
Hydra.)  or  the  Water-Snake.  Virgo  is  visible  in  the  evening, 
from  April  to  August. 

335.  The  Twins.  —  A  line  drawn  from  Delta  to  Beta  of  the 
Dipper,  and  prolonged  as  shown  in  Fig.  384,  passes  between 
two  bright  stars  called  Castor  and  Pollux.  The  latter  of  these 
is  usually  reckoned  as  a  first-magnitude  star.  These  are  the 


Fig.  383- 

principal  stars  of  the  constellation  Gemini,  or  the  Twins, 
which  is  shown  in  Fig.  385.  The  constellation  Cants  Minor, 
or  the  Little  Dog,  is  shown  in  the  lower  part  of  the  figure. 
There  are  two  conspicuous  stars  in  this  constellation,  the 
brightest  of  which  is  of  the  first  magnitude,  and  called  Procyon. 
The  region  to  which  we  have  now  been  brought  is  the 
richest  of  the  northern  sky,  containing  no  less  than  seven  first- 
magnitude  stars.  These  are  Sirius,  Procyon,  Pollux,  Capella, 
Aldebaran,  Betelgeuse,  and  Rigel.  They  are  shown  in  Fig.  386. 


ASTRONOMY. 


341 


Fig.  384- 

Betelgcuse  and   Rigel  are   in    tlie    constellation    Orion,   being 


Fig-  385- 


342  ASTRONOMY. 

about  equally  distant  to  the  north  and  south  from  the  three 
stars  forming  the  belt  of  Orion.  Betelgeuse  is  a  red  star. 
Sirius  is  the  brightest  star  in  the  heavens,  and  belongs  to  the 
constellation  Cants  Major,  or  the  Great  Dog.  It  lies  to  the 
east  of  the  belt  of  Orion.  Aldebaran  lies  at  about  the  same 
distance  to  the  west  of  the  belt.  It  is  a  red  star,  and  belongs 

o 


Fig.  386. 

to  the  constellation  Taurus,  or  the  Bull.  Capella  is  in  the 
constellation  Auriga,  or  the  Wagoner.  These  stars  are  visible 
in  the  evening,  from  about  December  to  April. 

336.  Orion  and  his  Dogs,  and  Taurus.  —  Orion  and  his 
Dogs  are  shown  in  Fig.  387,  and  Orion  and  T^aurus  in  Fig.  388. 
Aldebaran  marks  one  of  the  eyes  of  the  bull,  and  is  often  called 
the  Bull's  Eye.  The  irregular  V  in  the  face  of  the  bull  is 
called  the  Hyades,  and  the  cluster  on  the  shoulder  the  Pleiades. 


ASTRONOMY. 


343 


337.  The  Wagoner.  —  The  constellation  Auriga,  or  the 
Wagoner  (sometimes  called  the  Charioteer),  is  shown  in  Fig. 
389.  Capella  marks  the  Goat,  which  he  is  represented  as 
carrying  on  his  back,  and  the  little  right-angled  triangle  of 
stars  near  it  the  Kids.  The  five  chief  stars  of  this  constella- 
tion form  a  large,  irregular  pentagon.  Gamma  of  Auriga  is 


Fig.  387. 

also  Beta  of  Taurus,  and  marks  one  of  the  horns  of  the  Bull. 
338.  Pegasus,  Andromeda,  and  Perseits. —  A  line  drawn 
from  Polaris  near  to  Beta  of  Cassiopeia  will  lead  to  a  bright 
second-magnitude  star  at  one  corner  of  a  large  square  (Fig.  390). 
Alpha  belongs  both  to  the  Square  of  Pegasus  and  to  Androm- 
eda. Beta  and  Gamma,  which  are  connected  with  Alpha  in 
the  figure  by  a  dotted  line,  also  belong  to  Andromeda.  Algol, 


ASTRONOMY. 


Fig.  388. 

which  forms,  with  the  last-named  stars  and  with  the  Square  of 


Fig.  389- 


ASTRONOMY. 


345 


Pegasus,  an   asterism    similar   in    configuration   to   the    Great 


Fig.  391. 


346 


ASTRONOMY. 


Fig.  39?. 

Dipper,  belongs  to  Perseus.  Algenib,  which  is  reached  by 
bending  the  line  at  Gamma  in  the  opposite  direction,  is  the 
principal  star  of  Perseus. 


Fig.  393- 

Pegasus  is  shown  in  Fig.  391,  and  Andromeda  in  Fig.  392: 
Cetus,  the  Whale,  or  the  Sea  Monster,  shown  in  Fig.  393. 
belongs  to  the  same  mythological  group  of  constellations. 


ASTRONOMY. 


347 


339.  Scorpio,  Sagittarius,  and  Ophiuchus.  —  During  the 
summer  months  a  brilliant  constellation  is  visible,  called  Scor- 
pio, or  the  Scorpion.  The  configuration  of  the  chief  stars  of 
this  constellation  is  shown  in  Fig.  394.  They  bear  some 
resemblance  to  a  boy's  kite.  The  brightest  star  is  of  the  first 
magnitude,  and  called  Antares  (from  anti,  instead  of,  and  Ares, 
the  Greek  name  of  Mars),  because  it  rivals  Mars  in  redness. 
The  stars  in  the  tail  of  the  Scorpion  are  visible  in  our  latitude 
only  under  very  favorable  circumstances.  This  constellation 


Fig.  394- 

is  shown  in  Fig.  395,  together  with  Sagittarius  and  Ophiuchus. 
Sagittarius,  or  the  Archer,  is  to  the  east  of  Scorpio.  It  con- 
tains no  bright  stars,  but  is  easily  recognized  from  the  fact 
that  five  of  its  principal  stars  form  the  outline  of  an  inverted 
dipper,  which,  from  the  fact  of  its  being  partly  in  the  Milky- 
Way,  is  often  called  the  Milk  Dipper. 

Ophiuchus,  or  the  Serpent-Bearer,  is.  a  large  constellation, 
filling  all  the  space  between  the  head  of  Hercules  and  Scor- 
pio. It  is  difficult  to  trace,  since  it  contains  no  very  brilliant 
stars.  This  constellation  and  Libra,  or  the  Balances,  which  is 


348 


ASTRONOMY. 


Fig.  395- 


ASTRONOMY.  349 

the   zodiacal   constellation   to    the  west  of   Scorpio,  are  shown 
in  Fig.  396. 


Fig.  396. 
340.   Capricornus,  Aquarius,  and  tJie  Southern  Fish,  —  The 


Fig.  397. 

two  zodiacal  constellations  to  the  east  of  Sagittarius  are  Capri- 


35O  ASTRONOMY. 

cornus  and  Aquarius.  Capricornus  contains  three  pairs  of 
small  stars,  which  mark  the  head,  the  tail,  and  the  knees  of  the 
animal. 

Aquarius  is  marked  by  no  conspicuous  stars.  An  irregular 
line  of  minute  stars  marks  the  course  of  the  stream  of  water 
which  flows  from  the  Water-Bearer's  Urn  into  the  mouth  of  the 
Southern  Fish.  This  mouth  is  marked  by  the  first-magnitude 
star  Fomalhaut.  These  constellations  are  shown  in  Fig.  397. 

341.  Pisces  and  Aries.  —  The  remaining  zodiacal  constella- 
tions are  Pisces,  or  the  Fishes,  Aries,  or  the  Ram  (Fig.  398), 
and  Cancer,  or  the  Crab. 


Fig.  398. 

The  Fishes  lie  under  Pegasus  and  Andromeda,  but  contain 
no  bright  stars.  Aries  (between  Pisces  and  Taurus)  is  marked 
by  a  pair  of  stars  on  the  head,  —  one  of  the  second,  and  one 
of  the  third  magnitude.  Cancer  (between  Leo  and  Gemini}  has 
no  bright  stars,  but  contains  a  remarkable  cluster  of  small  stars 
called  Prcesepe,  or  the  Beehive. 

CLUSTERS. 

342.  The  Hyades.  —  The  Hyades  are  a  very  open  cluster  in 
the  face  of  Taurus  (334).  The  three  brightest  stars  of  this 
cluster  form  a  letter  V,  the  point  of  the  V  being  on  the  nose, 
and  the  open  ends  at  the  eyes.  This  cluster  is  shown  in  Fig. 


ASTRONOMY.  351 

399.     The  name,  according  to  the  most  probable  etymology, 

means     rainy ;     and 

they  are  said  to  have 

been    so    called    be- 
cause their  rising  was 

associated   with    wet 

weather.     They  were 

usually       considered 

the      daughters       of 

Atlas,  and  sisters  of 

the  Pleiades,  though 

sometimes      referred 

to   as  the   nurses   of 

Bacchus. 

343.   The  Pleiades. 

—  The  Pleiades  con- 
stitute   a    celebrated  Fi§-  399- 

group  of  stars,  or  a  miniature  constellation,  on  the  shoulder 

of  Taurus.  Hesiod 
mentions  them  as 
"  the  seven  virgins 
of  Atlas  born,"  and 
Milton  calls  them 
"the  seven  Atlantic 
sisters."  They  are 
referred  to  in  the 
Book  of  Job.  The 
Spaniards  term  them 
"  the  little  nanny- 
goats  ; "  and  they  are 
sometimes  called  "the 
hen  and  chickens." 

Usually  only  six 
stars  in  this  cluster 
can  be  seen  with  the 
naked  eye,  and  this 
fact  has  given  rise 
Fig'  4°0>  to  the  legend  of  the 

"  lost  Pleiad."     On  a  clear,  moonless  night,  however,  a  good 


352  ASTRONOMY. 

eye  can  discern  seven  or  eight  stars,  and  some  observers  have 
distinguished  as  many  as  eleven.     Fig.  400  shows  the  Pleiades 


Fig.  401. 

as    they  appear  to    the    naked    eye   under  the   most  favorable 
circumstances.     Fig.  401   shows  this   cluster  as  it  appears  in 


ASTRONOMY. 


353 


a  powerful  telescope.     With  such  an  instrument  more  than  five 
hundred  stars  are  visible. 

344.  Cluster  in  the  Sword-handle  of  Perseus.  —  This   is  a 
somewhat   dense    double    clus- 
ter.    It  is  visible  to  the  naked 

eye,  appearing  as  a  hazy  star. 
A  line  drawn  from  Algenib,  or 
Alpha  of  Perseus  (338),  to  Delta 
of  Cassiopeia  (330),  will  pass 
through  this  cluster  at  about 
two-thirds  the  distance  from 
the  former.  This  double  clus- 
ter is  one  of  the  most  brilliant 
objects  in  the  heavens,  with  a 
telescope  of  moderate  power. 

345.  Cluster  of  Hercules.  —  F'g-  4°2- 

The  celebrated  globular  cluster  of  Hercules  can  be  seen  only 
with  a  telescope  of  considerable  power,  and  to  resolve  it  into 


.   4°3- 


distinct  stars  (as  shown  in  Fig.  402)  requires  an  instrument  of 
the  very  highest  class. 


354 


ASTRONOMY. 


346.  Other  Chisters. 


Fig.  405. 

magnitude  being  immensely  numerous 
densation  of  light  at  the  centre. 


Fig.  404. 

Fig.  403  shows  a  magnificent  globular 
cluster  in  the  con- 
stellation Aquarius. 
Herschel  describes  it 
as  appearing  like  a 
heap  of  sand,  being 
composed  of  thou- 
sands of  stars  of  the 
fifteenth  magnitude. 

Fig.  404  shows  a 
cluster  in  the  con- 
stellation Toucan, 
which  Sir  John  Her- 
schel describes  as  a 
most  glorious  globu- 
lar cluster,  the  stars 
of  the  fourteenth 
There  is  a  marked  con- 


ASTRONOMY. 


355 


Fig.  405  shows  a  cluster  in  the  Centaur,  which,  according 
to  the  same  astrono- 
mer, is  beyond  com- 
parison the  richest  and 
largest  object  of  the 
kind  in  the  heavens, 
the  stars  in  it  being 
literally  innumerable. 
Fig.  406  shows  a  clus- 
ter in  Scorpio,  remarka- 
ble for  the  peculiar 
arrangement  of  its  com- 
ponent stars. 

Star     clusters      are 
especially  abundant   in 
the  region  of  the  Milky- 
Way,  the   law  of  their  Fig.  406. 
distribution  being  the  reverse  of  that  of  the  nebulae. 

DOUBLE  AND  MULTIPLE  STARS. 

347.  Double  Stars.  —  The  telescope  shows  that  many 
stars  which  appear  single  to  the  naked  eye  are  really  double, 
or  composed  of  a  pair  of  stars  lying  side  by  side.  There 
are  several  pairs  of  stars  in  the  heavens  which  lie  so  near 


Fig.  407.  Fig.  408. 

together  that  they  almost  seem  to  touch  when  seen  with 
the  naked  eye. 

Pairs  of  stars  are  not  considered  double  unless  the  com- 
ponents are  so  near  together  that  they  both  appear  in  the 


356 


ASTRONOMY. 


field  of  view  when  examined  with  a  telescope.  In  the 
majority  -of  the  pairs  classed  as  double  stars  the  distance 
between  the  components  ranges  from  half  a  second  to 
fifteen  seconds. 

Epsilon  Lyrce  is  a  good 
example  of  a  pair  of 
stars  that  can  barely  be 
separated  with  a  good 
eye.  Figs.  407  and  408 
show  this  pair  as  it  ap- 
pears in  telescopes  mag- 
nifying respectively  four 
and  fifteen  times ;  and 
Fig.  409  shows  it  as  seen 
in  a  more  powerful  tele- 
scope, in  which  each  of  Fig"  409' 
the  two  components  of  the  pair  is  seen  to  be  a  truly  double 
star. 

348.  Multiple  Stars. — When  a  star  is  resolved  into 
more  than  two  components  by  a  telescope,  it  is  called  a 
multiple  star.  Fig.  410  shows  a  triple  star  in  Pegasus. 


Fig.  410. 


Fig.  411. 


Fig.  411  shows  a  quadruple  star  in  Taurus.  Fig.  412 
shows  a  sextuple  star,  and  Fig.  413  a  septuple  star.  Fig. 
414  shows  the  celebrated  septuple  star  in  Orion,  called 
Theta  Orients,  or  the  trapezium  of  Orion. 


ASTRONOMY. 


357 


349.  Optically  Double  and  Multiple  Stars.  —  Two  or 
more  stars  which  are  really  very  distant  from  each  other, 
and  which  have  no  physical  connection  whatever,  may 
appear  to  be  near  together,  because  they  happen  to  lie  in 
the  same  direction,  one  behind  the  other.  Such  accidental 
combinations  are  called  optically  double  or  multiple  stars. 


12.  Fig.  413. 

350.  Physically    Double    and  Multiple    Stars.  —  In    the 
majority  of  cases  the  components  of  double  and  multiple 
stars    are    in    reality  comparatively  near  together,  and   are 
bound  together  by  gravity  into    a   physical    system.     Such 
combinations   are    called  physi- 
cally double  and  multiple  stars. 

The  components  of  these  sys- 
tems all  revolve  around  their 
common  centre  of  gravity.  In 
many  instances  their  orbits  and 
periods  of  revolution  have  been 
ascertained  by  observation  and  F'g-  4M- 

calculation.     Fig.  415  shows  the  orbit  of  one  of  the  com- 
ponents of  a  double  star  in  the  constellation  Hercules. 

351.  Colors  of  Double  and  Multiple  Stars.  —  The  com- 
ponents of  double  and  multiple  stars  are  often  highly  col- 
ored, and  frequently  the  components  of  the  same  system 
are  of   different  colors.     Sometimes   one   star  of  a  binary 
system  is  white,  and  the  other  red ;  and  sometimes  a  white 


ASTRONOMY. 


star  is  combined  with  a  blue  one.     Other  colors  found  in 
combination  in  these  systems  are  red  and  blue,  orange  and 
green,  blue  and  green,  yellow  and  blue,  yellow  and  red,  etc. 
If  these  double  and  multiple  stars  are  accompanied  by 


Fig.  415. 

planets,  these  planets  will  sometimes  have  two  or  more  suns 
in  the  sky  at  once.  On  alternate  days  they  may  have  suns 
of  different  colors,  and  perhaps  on  the  same  day  two 
suns  of  different  colors.  The  effect  of  these  changing 
colored  lights  on  the  landscape  must  be  very  remarkable. 

NEW  AND  VARIABLE  STARS. 

352.  Variable    Stars.  —  There    are    many    stars    which 
undergo  changes  of  brilliancy,  sometimes  slight,  but  occa- 
sionally very  marked.     These  changes  are   in   some   cases 
apparently  irregular,  and  in  others  periodic.     All  such  stars 
are  said  to  be  variable,  though  the  term  is  applied  espe- 
cially to  those  stars  whose  variability  is  periodic. 

353.  Algol.  —  Algol,  a  star  of  Perseus,  whose  position  is 


ASTRONOMY.  359 

shown  in  Fig.  416,  is  a  remarkable  variable  star  of  a  short 
period.  Usually  it  shines  as  a  faint  second-magnitude  star ; 
but  at  intervals  of  a  little  less  than  three  days  it  fades  to 
the  fourth  magnitude  for  a  few  hours,  and  then  regains  its 
former  brightness.  These  changes  were  first  noticed  some 
two  centuries  ago,  but  it  was  not  till  1782  that  they  were 
accurately  observed.  The  period  is  now  known  to  be  two 
days,  twenty  hours,  forty-nine  minutes.  It  takes  about  four 
hours  and  a  half  to  fade  away,  and  four  hours  more  to 
recover  its  brilliancy.  Near  the  beginning  and  end  of  the 
variations,  the  change  is  very 
slow,  so  that  there  are  not  more 
than  five  or  six  hours  during 
which  an  ordinary  observer 
would  see  that  the  star  was 
less  bright  than  usual. 

This  variation  of  light  was  at 
first  explained  by  supposing  that 
a  large  dark  planet  was  revolv- 
ing round  Algol,  and  passed 
over  its  face  at  every  revolution, 
thus  cutting  off  a  portion  of  its 
light ;  but  there  are  small  irregu- 
larities in  the  variation,  which 
this  theory  does  not  account  for.  Fig.  416. 

354.  Mira. —  Another  remarkable  variable  star  is  Omicron 
Ceti,  or  Mira  (that  is,  the  wonderful  star) .  It  is  generally 
invisible  to  the  naked  eye  ;  but  at  intervals  of  about  eleven 
months  it  shines  forth  as  a  star  of  the  second  or  third 
magnitude.  It  is  about  forty  days  from  the  time  it  becomes 
visible  until  it  attains  its  greatest  brightness,  and  is  then 
about  two  months  in  fading  to  invisibility ;  so  that  its 
increase  of  brilliancy  is  more  rapid  than  its  waning.  Its 
period  is  quite  irregular,  ranging  from  ten  to  twelve  months ; 
so  that  the  times  of  its  appearance  cannot  be  predicted 


360  ASTRONOMY. 

with  certainty.  Its  maximum  brightness  is  also  variable, 
being  sometimes  of  the  second  magnitude,  and  at  others 
only  of  the  third  or  fourth. 

355.  Eta  Argus.  —  Perhaps  the  most  extraordinary  varia- 
ble star  in  the  heavens  is  Eta  Argus,  in  the  constellation 
Argo,  or  the  Ship,  in  the  southern  hemisphere  (Fig.  417). 
The  first  careful  observations  of  its  variability  were  made 
by. Sir  John  Herschel  while  at  the  Cape  of  Good  Hope. 
He  says,  "It  was  on  the  i6th  of  December,  1837,  that, 
resuming  the  photometrical  comparisons,  my  astonishment 
was  excited  by  the  appearance  of  a  new  candidate  for  dis- 
tinction among  the  very  brightest  stars  of  the  first  magni- 
tude in  a  part  of  the  heavens 
where,  being  perfectly  familiar 
with  it,  I  was  certain  that  no 
such  brilliant  object  had  before 
been  seen.  After  a  momentary 
hesitation,  the  natural  conse- 
quence of  a  phenomenon  so 
utterly  unexpected,  and  refer- 
ring to  a  map  for  its  configura- 
tion with  other  conspicuous 
Fig.  417-  stars  in  the  neighborhood,  I 

became  satisfied  of  its  identity  with  my  old  acquaintance, 
Eta  Argus.  Its  light  was,  however,  nearly  tripled.  While 
yet  low,  it  equalled  Rigel,  and,  when  it  attained  some 
altitude,  was  decidedly  greater."  It  continued  to  increase 
until  Jan.  2,  1838,  then  faded  a  little  till  April  following, 
though  it  was  still  as  bright  as  Aldebaran.  In  1842  and 
1843  ft  blazed  up  brighter  than  ever,  and  in  March  of  the 
latter  year  was  second  only  to  Sinus.  During  the  twenty- 
five  years  following  it  slowly  but  steadily  diminished.  In 
1867  it  was  barely  visible  to  the  naked  eye;  and  the  next 
year  it  vanished  entirely  from  the  unassisted  view,  and  has 
not  yet  begun  to  recover  its  brightness.  The  curve  in 


ASTRONOMY.  361 

Fig.  418  shows  the  change  in  brightness  of  this  remarkable 
star.  The  numbers  at  the  bottom  show  the  years  of  the 
century,  and  those  at  the  side  the  brightness  of  the  star. 

356.  New  Stars.  —  In  several  cases  stars  have  suddenly 
appeared,  and  even  become  very  brilliant ;  then,  after  a 
longer  or  shorter  time,  they  have  faded  away  and  disap- 
peared. Such  stars  are  called  new  or  temporary  stars. 
For  a  time  it  was  supposed  that  such  stars  were  actually 
new.  They  are  now,  however,  classified  by  astronomers 
among  the  variable  stars,  their  changes  being  of  a  very 
irregular  and  fitful  character.  There  is  scarcely  a  doubt 
that  they  were  all  in  the  heavens  as  very  small  stars  before 


they  blazed  forth  in  so  extraordinary  a  manner,  and  that 
they  are  in  the  same  places  still.  There  is  a  wide  difference 
between  these  irregular  variations,  or  the  breaking-forth  of 
light  on  a  single  occasion  in  the  course  of  centuries,  and 
the  regular  and  periodic  changes  in  the  case  of  a  star  like 
Algol ;  but  a  long  series  of  careful  observation  has  resulted 
in  the  discovery  of  stars  of  nearly  every  degree  of  irregu- 
larity between  these  two  extremes.  Some  of  them  change 
gradually  from  one  magnitude  to  another,  in  the  course  of 
years,  without  seeming  to  follow  any  law  whatever ;  while 
in  others  some  slight  tendency  to  regularity  can  be  traced. 
Eta  Argus  may  be  regarded  as  a  connecting  link  between 
new  and  variable  stars. 

357.   Tycho    Brake's    Star.  —  An    apparently   new   star 


362  ASTRONOMY. 

suddenly  appeared  in  Cassiopeia  in  1572.  It  was  first  seen 
by  Tycho  Brahe,  and  is  therefore  associated  with  his  name. 
Its  position  in  the  constellation  is  shown  in  Fig.  419.  It 
was  first  seen  on  Nov.  n,  when  it  had  already  attained  the 
first  magnitude.  It  became  rapidly  brighter,  soon  rivalling 
Venus  in  splendor,  so  that  good  eyes  could  discern  it  in 
full  daylight.  In  December  it  began  to  wane,  and  gradu- 
ally faded  until  the  following  May,  when  it  disappeared 
entirely. 

A  star  showed  itself  in  the  same  part  of  •  the  heavens  in 
945  and  in  1264.  If  these  were  three  appearances  of  the 

same  star,  it  must 
be  reckoned  as  a 
periodic  star  with 
a  period  of  a  little 
more  than  three 
hundred  years. 

358.  Kepler's 
Star.  —  In  1604  a 
new  star  was  seen 
in  the  constellation 
Ophiuchus.  It  was 
first  noticed  in 
Fig-4'9'  October  of  that 

year,  when  it  was  of  the  first  magnitude.  In  the  following 
winter  it  began  to  fade,  but  remained  visible  during  the 
whole  year  1605.  Early  in  1606  it  disappeared  entirely. 
A  very  full  history  of  this  star  was  written  by  Kepler. 

One  of  the  most  remarkable  things  about  this  star  was 
its  brilliant  scintillation.  According  to  Kepler,  it  displayed 
all  the  colors  of  the  rainbow,  or  of  a  diamond  cut  with 
multiple  facets,  and  exposed  to  the  rays  of  the  sun.  It  is 
thought  that  this  star  also  appeared  in  393.  798,  and  1203 ; 
if  so,  it  is  a  variable  star  with  a  period  of  a  little  over  four 
hundred  years. 


ASTRONOMY.  363 

359.  New  Star  of  1866.  —  The   most  striking   case   of 
this  kind  in  recent  times  was  in  May,  1866,  when  a  star 
of  the    second   magnitude    suddenly   appeared    in    Corona 
Borealis.     On    the    nth   and   i2th    of  that   month  it  was 
observed  independently  by  at  least  five  observers  in  Europe 
and  America.     The  fact  that  none  of  these  new  stars  were 
noticed  until  they  had  nearly  or  quite  attained  their  greatest 
brilliancy  renders  it  probable  that  they  all  blazed  up  very 
suddenly. 

360.  Cause  of  the  Variability  of  Stars.  —  The   changes   in 
the  brightness  of  variable  and  temporary  stars  are  probably 
due  to  operations  similar  to  those  which  produce  the  spots  and 
prominences   in    our  sun.     We   have    seen  (188)  that  the  fre- 
quency of  solar  spots  shows  a  period  of  eleven  years,  during 
one  portion  of  which  there  are  few  or  no  spots  to  be  seen,  while 
during  another  portion  they  are  numerous.     If  an  observer  so 
far  away  as  to  see  our  sun  like  a  star  could  from  time  to  time 
measure  its  light  exactly,  he  would  find  it  to  be  a  variable  star 
with  a  period  of  eleven  years,  the  light  being  least  when  we 
see  most  spots,  and  greatest  when  few  are  visible.     The  varia- 
tion would  be  slight,  but  it  would  nevertheless  exist.      Now, 
we  have  reason  to  believe  that  the  physical  constitution  of  the 
sun  and  the  stars  is  of  the  same  general  nature.     It  is  there- 
fore probable,  that,  if  we  could  get  a  nearer  view  of  the  stars, 
we  should  see  spots  on  their  disks  as  we  do  on  the  sun.     It 
is  also  likely  that  the  varying  physical  constitution  of  the  stars 
might  give  rise  to  great  differences  in  the  number  and  size  of 
the  spots ;  so  that  the  light  of  some  of  these  suns  might  vary 
to  a  far  greater  degree  than  that  of  our  own  sun  does.     If  the 
variations  had  a  regular  period,  as  in  the  case  of  our  sun,  the 
appearances  to  a  distant  observer  would  be  precisely  what  we 
see  in  the  case  of  a  periodic  variable  star. 

The  spectrum  of  the  new  star  of  1866  was  found  to  be  a 
continuous  one,  crossed  by  bright  lines,  which  were  apparently 
due  to  glowing  hydrogen.  The  continuous  spectrum  was  also 
crossed  by  dark  lines,  indicating  that  the  light  had  passed 
through  an  atmosphere  of  comparatively  cool  gas.  Mr.  Huggins 


364  ASTRONOMY. 

infers  from  this  that  there  was  a  sudden  and  extraordinary  out- 
burst of  hydrogen  gas  from  the  star,  which  by  its  own  light, 
as  well  as  by  heating  up  the  whole  surface  of  the  star,  caused 
the  extraordinary  increase  of  brilliancy.  Now,  the  spectro- 
scope shows  that  the  red  flames  of  the  solar  chromosphere 
(197)  are  largely  composed  of  hydrogen;  and  it  is  not  unlikely 
that  the  blazing-forth  of  this  star  arose  from  an  action  similar 
to  that  which  produces  these  flames,  only  on  an  immensely 
larger  scale. 

DISTANCE  OF  THE  STARS. 

361.  Parallax  of  the  Stars.  —  Such   is   the    distance  of 
the  stars,  that  only  in  a  comparatively  few  instances  has  any 
displacement  of  these  bodies  been  detected  when  viewed 
from  opposite  parts  of  the  earth's  orbit,  that  is,  from  points 
a  hundred  and  eighty-five  million  miles  apart ;  and  in  no 
case  can  this  displacement  be  detected  except  by  the  most 
careful  and  delicate  measurement.     Half  of  the  above  dis- 
placement, or  the   displacement  of  the   star  as   seen  from 
the  earth  instead  of  the  sun,  is  called  the  parallax  of  the 
star.     In  no  case  has  a  parallax  of  one  second  as  yet  been 
detected. 

362.  The  Distance  of  the  Stars.  —  The  distance  of  a  star 
whose  parallax  is  one  second  would  be  206,265  times  the 
distance  of  the  earth  from  the  sun,  or  about  nineteen  million 
million  miles.     It  is  quite  certain  that  no  star  is  nearer  than 
this  to  the  earth.     Light  has  a  velocity  which  would  carry 
it  seven  times  and  a  half  around  the  earth  in  a  second ;  but 
it  would  take  it  more  than  three  years  to  reach  us  from 
that  distance.     Were  all  the  stars  blotted  out  of  existence 
to-night,  it  would  be  at  least  three  years  before  we  should 
miss  a  single  one. 

Alpha  Centauri,  the  brightest  star  in  the  constellation 
of  the  Centaur,  is,  so  far  as  we  know,  the  nearest  of  the 
fixed  stars.  It  is  estimated  that  it  would  take  its  light  about 
three  years  and  a  half  to  reach  us.  It  has  also  been  esti- 


ASTRONOMY.  365 

mated  that  it  would  take  light  over  sixteen  years  to  reach 
us  from  Sirius,  about  eighteen  years  to  reach  us  from  Vega, 
about  twenty-five  years  from  Arcturus,  and  over  forty  years 
from  the  Pole-Star.  In  many  instances  it  is  believed  that 
it  would  take  the  light  of  stars  hundreds  of  years  to  make 
the  journey  to  our  earth,  and  in  some  instances  even  thou- 
sands of  years. 

PROPER  MOTION  OF  THE  STARS. 

363.    Why  the  Stars  appear  Fixed.  —  The  stars  seem  to 
retain  their  relative  positions  in  the*  heavens  from  year  to 


Fig.  420. 

year,  and  from  age  to  age ;  and  hence  they  have  come 
universally  to  be  denominated  as  fixed.  It  is,  however,  now 
well  known  that  the  stars,  instead  of  being  really  stationary, 
are  moving  at  the  rate  of  many  miles  a  second ;  but  their 
distance  is  so  enormous,  that,  in  the  majority  of  cases,  it 
would  be  thousands  of  years  before  this  rate  of  motion 
would  produce  a  sufficient  displacement  to  be  noticeable 
to  the  unaided  eye. 


366 


ASTRONOMY. 


364.  Secular  Displacement  of  the  Stars. — Though  the 
proper  motion  of  the  stars  is  apparently  slight,  it  will,  in 
the  course  of  many  ages,  produce  a  marked  change  in  the 
configuration  of  the  stars.  Thus,  in  Fig.  420,  the  left-hand 
portion  shows  the  present  configuration  of  the  stars  of 
the  Great  Dipper.  The  small  arrows  attached  to  the  stars 
show  the  direction  and  comparative  magnitudes  of  their 
motion.  The  right-hand  portion  of  the  figure  shows  these 


Fig.  421. 

stars  as  they  will  appear  thirty-six  thousand  years  from  the 
present  time. 

Fig.  421  shows  in  a  similar  way  the  present  configura- 
tion and  proper  motion  of  the  stars  of  Cassiopeia,  and 
also  these  stars  as  they  will  appear  thirty-six  thousand  years 
hence. 

Fig.  422  shows  the  same  for  the  constellation  Orion. 

365.  The  Secular  Motion  of  the  Sun.  —  The  stars  in  all 
parts  of  the  heavens  are  found  to  move  in  all  directions 
and  with  all  sorts  of  velocities.  When,  however,  the  motions 


ASTRONOMY. 


of  the  stars  are  averaged,  there  is  found  to  be  an  apparent 
proper  motion  common  to  all  the  stars.     The  stars  in  the 


Fig.  4=2. 

neighborhood  of  Hercules  appear  to  be  approaching  us, 
and  those  in  the 
opposite  part  of 
the  heavens  ap- 
pear to  be  re- 
ceding from  us. 
In  other  words, 
all  the  stars 
appear  to  be 
moving  away 
from  Hercules, 
and  towards  the 
opposite  part  of 
the  heavens.  Fig.  423. 

This  apparent  motion  common  to  all  the  stars  is  held  by 
astronomers    to   be    due    to    the    real    motion    of    the    sun 


368  ASTRONOMY. 

through  space.  The  point  in  the  heavens  towards  which 
our  sun  is  moving  at  the  present  time  is  indicated  by  the 
small  circle  in  the  constellation  Hercules  in  Fig.  423.  As 
the  sun  moves,  he  carries  the  earth  and  all  the  planets  along 
with  him.  Fig.  424  shows  the  direction  of  the  sun's  motion 

with  reference  to  the  eclip- 
tic and  to  the  axis  of  the 
earth.  Fig.  425  shows  the 
earth's  path  in  space ;  and 
Fig.  426  shows  the  paths  of 
the  earth,  the  moon,  Mer- 
cury, Venus,  and  Mars  in 
space. 

Whether  the  sun  is  actu- 

Fig.  424.  ally    moving    in    a    straight 

line,  or  around  some  distant  centre,  it  is  impossible  to  deter- 
mine at  the  present  time.  It  is  estimated  that  the  sun  is 
moving  along  his  path  at  the  rate  of  about  a  hundred  and 
fifty  million  miles  a  year.  This  is  about  five-sixths  of  the 
diameter  of  the  earth's 
orbit. 

366.  Star -Drift.  —  In 
several  instances,  groups 
of  stars  have  a  common 
proper  motion  entirely  dif- 
ferent from  that  of  the 
stars  around  and  among 
them.  Such  groups  proba- 
bly form  connected  sys- 
tems, in  the  motion  of  Flg-  425- 
which  all  the  stars  are  carried  along  together  without  any 
great  change  in  their  relative  positions.  The  most  re- 
markable case  of  this  kind  occurs  in  the  constellation 
Taurus.  A  large  majority  of  the  brighter  stars  in  the 
region  between  Aldebaran  and  the  Pleiades  have  a  common 


ASTRONOMY. 


369 


proper  motion  of  about  ten  seconds  per  century  towards 
the  east.  Proctor  has  shown  that  five  out  of  the  seven 
stars  which  form  the  Great  Dipper  have  a  common  proper 


Fig.  426. 

motion,  as  shown  in  Fig.  427  (see  also  Fig.  420).     He  pro- 
poses for  this  phenomenon  the  name  of  Star-Drift. 


*-.. 


Fig.  427- 


367.  Motion  of  Stars  along  the  Line  of  Sight.  —  A  motion 
of  a  star  in  the  direction  of  the  line  of  sight  would  produce 
no  displacement  of  the  star  that  could  be  detected  with  the 


3/O  ASTRONOMY. 

telescope ;  but  it  would  cause  a  change  in  the  brightness  of 
the  star,  which  would  become  gradually  fainter  if  moving  from 
us,  and  brighter  if  approaching  us.  Motion  along  the  line 
of  sight  has,  however,  been  detected  by  the  use  of  the  tele- 
spectroscope  (152),  owing  to  the  fact  that  it  causes  a  displace- 
ment of  the  spectral  lines.  As  has  already  been  explained 
(169),  a  displacement  of  a  spectral  line  towards  the  red  end 
of  the  spectrum  indicates  a  motion  away  from  us,  and  a  dis- 
placement towards  the  violet  end,  a  motion  towards  us. 

By  means  of  these  displacements  of  the  spectral  lines, 
Huggins  has  detected  motion  in  the  case  of  a  large  number 
of  stars,  and  calculated  its  rate  :  — 

STARS    RECEDING   FROM    US. 

Sirius 20  miles  per  second. 

Betelgeuse 22  miles  per  second. 

Rigel 15  miles  per  second. 

Castor 25  miles  per  second. 

Regulus 15  miles  per  second. 

STARS    APPROACHING   US. 

Arcturus 55  miles  per  second. 

Vega 50  miles  per  second. 

Deneb 39  miles  per  second. 

Pollux 49  miles  per  second. 

Alpha  Ursae  Majoris    ...  46  miles  per  second. 

These  results  are  confirmed  by  the  fact  that  the  amount 
of  motion  indicated  is  about  what  we  should  expect  the 
stars  to  have,  from  their  observed  proper  motions,  combined 
with  their  probable  distances.  Again :  the  stars  in  the 
neighborhood  of  Hercules  are  mostly  found  to  be  approach- 
ing the  earth,  and  those  which  lie  in  the  opposite  direction 
to  be  receding  from  it ;  which  is  exactly  the  effect  which 
would  result  from  the  sun's  motion  through  space.  The 
five  stars  in  the  Dipper,  which  have  a  common  proper 


ASTRONOMY.  3/1 

motion,  are  also  found  to  have  a  common  motion  in  the 
line  of  sight.  But  the  displacement  of  the  spectral  lines 
is  so  slight,  and  its  measurement  so  difficult,  that  the  veloci- 
ties in  the  above  table  are  to  be  accepted  as  only  an 
approximation  to  the  true  values. 

CHEMICAL  AND  PHYSICAL  CONSTITUTION  OF  THE  STARS. 

368.  The   Constitution   of  the  Stars  Similar  to  that  of 
the  Sun.  —  The  stellar  spectra  bear  a  general  resemblance 
to  that  of  the  sun,  with  characteristic  differences.     These 
spectra   all    show   Fraunhofer's    lines,   which    indicate    that 
their  luminous  surfaces  are  surrounded  by  atmospheres  con- 
taining absorbent  vapors,  as  in  the  case  of  the  sun.     The 
positions  of  these  lines  indicate  that  the  stellar  atmospheres 
contain  elements  which  are   also   found  in  the   sun's,  and 
on  the  earth. 

369.  Four  Types  of  Stellar  Spectra.  —  The    spectra   of 
the  stars  have  been  carefully  observed  by  Secchi  and  Hug- 
gins.     They  have  found  that  stellar  spectra  may  be  reduced 
to  four  types,  which  are  shown  in  Fig.  428.     In  the  spec- 
trum of  Sirius,  a  representative  of  Type  /.,  very  few  lines 
are  represented  ;  but  the  lines  are  very  thick. 

Next  we  have  the  solar  spectrum,  which  is  a  repre- 
sentative of  Type  II.,  one  in  which  more  lines  are  rep- 
resented. In  Type  III.  fluted  spaces  begin  to  appear, 
and  in  Type  IV.,  which  is  that  of  the  red  stars,  nothing 
but  fluted  spaces  is  visible  ;  and  this  spectrum  shows  that 
something  is  at  work  in  the  atmosphere  of  those  red 
stars  different  from  what  there  is  in  the  simpler  atmosphere 
of  Type  L 

Lockyer  holds  that  these  differences  of  spectra  are  due 
simply  to  differences  of  temperature.  According  to  him, 
the  red  stars,  which  give  the  fluted  spectra,  are  of  the 
lowest  temperature ;  and  the  temperature  of  the  stars  of 


372  ASTRONOMY. 

the   different    types  gradually  rises    till  we    reach   the    first 
type,  in  which  the  temperature    is    so    high    that   the    dis- 


Fig.  428. 

sociation    (161)    of  the    elements    is    nearly    if    not   quite 
complete. 


ASTRONOMY.  373 

III.      NEBULA. 

CLASSIFICATION  OF  NEBUIJE. 

370.  Planetary  Nebula.  —  Many  nebulae  (328)  present  a 
well-defined  circular  disk,  like  that  of  a  planet,  and  are  there- 
fore called  planetary  nebulae.     Specimens  of  planetary  neb- 
ulae are  shown  in  Fig.  429. 

371.  Circular    and    Elliptical    Nebula. — While    many 
nebulae  are  circular  in  form,  others  are  elliptical.     The  for- 
mer are    called    circular  nebulae,  and    the    latter  elliptical 
nebulae.     Elliptical  nebulae  have  been  discovered  of  every 
degree  of  eccentricity.     Examples  of  various  circular  and 
elliptical  nebulae  are  given  in  Fig.  430. 


Fig.  429. 

372.  Annular  Neb  u  ha. —  Occasionally  ring-shaped  nebu- 
lae   have    been    observed,   sometimes   with,   and   sometimes 
without,  nebulous  matter  within   the  ring.     They  are  called 
annular  nebulae.     They  are  both  circular  and  elliptical  in 
form.     Several  specimens  of  this  class  of  nebulae  are  given 
in  Fig.  431. 

373.  Nebulous  Stars.  —  Sometimes  one  or  more  minute 
stars  are   enveloped   in    a   nebulous    haze,   and    are    hence 
called  nebulous  stars.     Several  of  these  nebulae  are   shown 
in  Fig.  432. 

374.  Spiral   Nebulcc. — Very    many    nebulae    disclose    a 
more    or  less    spiral    structure,   and    are    known    as   spiral 
nebulae.     They  are  illustrated  in  Fig.  433.     There  are,  how- 


374 


ASTRONOMY. 


Fig.  43 


ASTRONOMY. 


375 


ever,  a  great  variety  of  spiral  forms.     We  shall  have  occa- 
sion to  speak  of  these  nebulae  again  (381-383). 

375.  Double  and  Multiple  Nebula.  —  Many  double   and 


Fig.  432. 

multiple  nebulae  have  been  observed,  some  of  which  are 
represented  in  Fig.  434. 

Fig.  435   shows  what  appears   to   be  a  double  annular 
nebula.     Fig.   436    gives   two    views  .of  a   double    nebula. 


Fig-  433- 

The  change  of  position  in  the  components  of  this  double 
nebula  indicates  a  motion  of  revolution  similar  to  that  of 
the  components  of  double  stars. 


376 


ASTRONOMY. 


IRREGULAR  NEBUUE. 

376.  Irregular  Forms.  —  Besides  the  more  or  less  regu- 
lar forms  of  nebulae  which  have  been  classified  as  indicated 


Fig.  434- 

above,  there  are  many  of  very  irregular  shapes,  and  some 
of  these  are  the  most  remarkable  nebulae  in  the  heavens. 
Fig.  437  shows  a  curiously  shaped  nebula,  seen  by  Sir 

John  Herschel  in  the  southern 
heavens ;  and  Fig.  438,  one  in 
Taurus,  known  as  the  Crab 
nebula. 

377.   The   Great  Nebula    of 
Andromeda.  —  This  is   one   of 
the  few  nebulae  that  are  visible 
to  the  naked  eye.     We  see  at 
Fig.  435-  a  glance  that  it  is  not  a  star, 

but  a  mass  of  diffused  light.  Indeed,  it  has  sometimes 
been  very  naturally  mistaken  for  a  comet.  It  was  first 
described  by  Marius  in  1614,  who  compared  its  light  to 


ASTRONOMY. 


377 


that  of  a  candle  shining  through  horn.  This  gives  a  very 
good  idea  of  the  impression  it  produces,  which  is  that  of 
a  translucent  object  illuminated  by  a  brilliant  light  behind 
it.  With  a  small 
telescope  it  is  easy 
to  imagine  it  to  be 
a  solid  like  horn ; 
but  with  a  large  one 
the  effect  is  more 
like  fog  or  mist  with 
a  bright  body  in  its 
midst.  Unlike  most 
of  the  nebulae,  its 
spectrum  is  a  con- 
tinuous one,  similar  Fig.  436. 
to  that  from  a  heated  solid,  indicating  that  the  light 
emanates,  not  from  a  glowing  gas,  but  from  matter  in  the 
solid  or  liquid  state.  This  would  suggest  that  it  is  really 


Fig-  437- 

an  immense  star-cluster,  so  distant  that  the  highest  tele- 
scopic power  cannot  resolve  it ;  yet  in  the  largest  telescopes 
it  looks  less  resolvable,  and  more  like  a  gas,  than  in 
those  of  moderate  size.  If  it  is  really  a  gas,  and  if  the 


378  ASTRONOMY. 

spectrum  is  continuous  throughout  the  whole  extent  of 
the  nebula,  either  it  must  shine  by  reflected  light,  or  the 
gas  must  be  subjected  to  a  great  pressure  almost  to  its 
outer  limit,  which  is  hardly  possible.  If  the  light  is  re- 
flected, we  cannot  determine  whether  it  comes  from  a  single 


Fig.  438. 

bright  star,  or  a  number  of  small  ones  scattered  through 
the  nebula. 

With  a  small  telescope  this  nebula  appears  elliptical,  as 
in  Fig.  439.  Fig.  440  shows  it  as  it  appeared  to  Bond,  in 
the  Cambridge  refractor. 

378.  The  Great  Nebula  of  Orion. — The  nebula  which, 
above  all  others,  has  occupied  the  attention  of  astrono- 


ASTRONOMY. 


379 


Fig.  439- 


Fig.  440. 


380 


ASTRONOMY 


Fig.  441. 


ASTRONOMY. 


mers,  and  excited  the  wonder  of  observers,  is  the  great 
nebula  of  Orion,  which  surrounds  the  middle  star  of  the 
three  which  form  the  sword  of  Orion.  A  good  eye  will  per- 
ceive that  this  star,  instead  of  looking  like  a  bright  point, 
has  a  hazy  appearance,  due  to  the  surrounding  nebula. 
This  object  was  first  described  by  Huyghens  in  1659,  as 
follows  :  — 

"  There  is  one  phenomenon  among  the  fixed  stars  worthy 
of  mention,  which,  so   far  as    I    know,  has   hitherto  been 


Fig.  44=- 

noticed  by  no  one,  and  indeed  cannot  be  well  observed 
except  with  large  telescopes.  In  the  sword  of  Orion  are 
three  stars  quite  close  together.  In  1656,  as  I  chanced 
to  be  viewing  the  middle  one  of  these  with  the  telescope, 
instead  of  a  single  star,  twelve  showed  themselves  (a  not 
uncommon  circumstance).  Three  of  these  almost  touched 
each  other,  and  with  four  others  shone  through  a  nebula, 
so  that  the  space  around  them  seemed  far  brighter  than  the 
rest  of  the  heavens,  which  was  entirely  clear,  and  appeared 


382 


ASTRONOMY. 


quite  black ;  the  effect  being  that  of  an  opening  in  the  sky, 

through  which  a  brighter  region  was  visible." 

The  representation 
of  this  nebula  in  Fig. 
441  is  from  a  drawing 
made  by  Bond.  In 
brilliancy  and  variety 
of  detail  it  exceeds 
any  other  nebula  visi- 
ble in  the  northern 
hemisphere.  In  its 
centre  are  four  stars, 
easily  distinguished  by 
Fig.  443.  a  small  telescope  with 

a  magnifying   power  of   forty    or   fifty,   together  with   two 


Fig.  444. 

smaller  ones,  requiring  a  nine-inch  telescope  to  be  well  seen. 
Besides  these,  the  whole  nebula  is  dotted  with  stars. 


ASTRONOMY.  383 

In  the  winter  of  1864-65  the  spectrum  -of  this  nebula 
was  examined  independently  by  Secchi  and  Huggins,  who 
found  that  it  consisted  of  three  bright  lines,  and  hence 
concluded  that  the  nebula  was  composed,  not  of  stars,  but 
of  glowing  gas.  The  position  of  one  of  the  lines  was  near 
that  of  a  line  of  nitrogen,  while  another  seemed  to  coin- 
cide with  a  hydrogen  line.  This  would  suggest  that  the 
nebula  is  a  mixture  of  hydrogen  and  nitrogen  gas ;  but  of 
this  we  cannot  be  certain. 

379.  The  Nebula  in  Argus.  —  There   is   a   nebula   (Fig. 
442)     surrounding 

the  variable  star 
Eta  Argus  (355), 
which  is  remarka- 
ble as  exhibiting 
variations  of  bright- 
ness and  of  out- 
line. 

In  many  other 
nebulae,  changes 
have  been  suspect- 
ed ;  but  the  indis- 
tinctness of  outline 
which  characterizes 

most  of  these  ob-    ~  Fig.  445. 

jects,  and  the  very  different  aspect  they  present  in  telescopes 
of  different  powers,  render  it  difficult  to  prove  a  change 
beyond  a  doubt. 

380.  The.   Dumb-Bell  Nebula.  —  This  nebula  was  named 
from  its  peculiar  shape.     It   is   a  good   illustration  of  the 
change   in   the  appearance  of  a  nebula  when  viewed  with 
different  magnifying  powers.     Fig.  443  shows  it  as  it  ap- 
peared in  Herschel's  telescope,  and  Fig.  444  as  it  appears 
in  the  great  Parsonstown  reflector  (20). 


ASTRONOMY. 


SPIRAL  NEBULJE. 


381.   The  Spiral  Nebula  in  Canes  Venatici.  —  The  great 
spiral  nebula  in  the   constellation    Canes   Venatici,  or  the 


Fig.  446. 

Hunting-Dogs,  is  one  of  the  most  remarkable  of  its  class. 
Fig.  445  shows  this  nebula  as  it  appeared  in  Herschel's 
telescope,  and  Fig.  446  shows  it  as  it  appears  in  the  Par- 
sonstown  reflector. 


ASTRONOMY.  385 

382.   Condensation  of  Nebula. — The  appearance  of  the 


Fig.  447. 

nebula  just  mentioned  suggests  a  body  rotating  on  its  axis, 
and  undergoing  condensation  at  the  same  time. 


Fig.  448. 

It  is  now  a  generally  received  theory  that  nebulae  are  the 
material  out  of  which  stars  are  formed.     According  to  this 


386  ASTRONOMY. 

theory,  tne  stars  originally  existed  as  nebulae,  and  all  nebulae 
will  ultimately  become  condensed  into  stars. 


Fig.   449. 

383.    Other  Spiral  Nebula.  —  Fig.  447  represents  a  spiral 


ASTRONOMY. 


387 


nebula  of  the  Great  Bear.     This  nebula  seems  to  have  sev- 
eral centres  of  condensation.     Fig.  448  is  a  view  of  a  spiral 


Fig.  450. 

nebula  in  Cepheus,  and  Fig.  449  of  a  singular  spiral  nebula 
in  the  Triangle.     This  also  appears  to  have  several  points 


Fig-  451' 

of   condensation.     Figs.   450  and  451   represent  oval   and 
elliptical  nebulae  having  a  spiral  structure. 


388 


ASTRONOMY. 


ASTRONOMY.  389 

THE  MAGELLANIC  CLOUDS. 

384.  Situation  and  General  Appearance  of  the  Magel- 
lanic  Clouds.  —  The  Magellanic  clouds  are  two  nebulous- 
looking  bodies  near  the 
southern  pole  of  the  heav- 
ens, as  shown  in  the  right- 
hand  portion  of  Fig.  452. 
In  the  appearance  and 
brightness  of  their  light 
they  resemble  portions  of 
the  Milky- Way. 

The  larger  of  these 
clouds  is  called  the  Nit- 
becula  Major.  It  is  visi- 
ble to  the  naked  eye  in 
strong  moonlight,  and  cov- 
ers a  space  about  two  Fig.  453. 
hundred  times  the  surface  of  the  moon.  It  is  shown  in 

Fig.  453.  The  smaller 
cloud  is  called  the  Nu- 
bccula  Minor.  It  has 
only  about  a  fourth  the 
extent  of  the  larger  cloud, 
and  is  considerably  less 
brilliant.  It  is  visible  to 
the  naked  eye,  but  it  dis- 
appears in  full  moonlight. 
This  cloud  is>  shown  in 
Fig.  454.  The  region 
around  this  cloud  is  sin- 
gularly bare  of  stars ;  but 
Fig.  454.  the  magnificent  cluster  of 

Toucan,  already  described   (346),  is  near,  and  is  shown  a 
little  to  the  right  of  the  cloud  in  the  figure. 


390  ASTRONOMY. 

385.  Structure  of  the  Nubecula. —  Fig.  455  shows  the 
structure  of  these  clouds  as  revealed  by  a  powerful  tele- 
scope. The  general  ground  of  both  consists  of  large  tracts 
and  patches  of  nebulosity  in  every  stage  of  resolution,  — 
from  that  which  is  irresolvable  with  eighteen  inches  of 
reflecting  aperture,  up  to  perfectly  separated  stars,  like  the 
Milky- Way  and  clustering  groups.  There  are  also  nebulae 
in  abundance,  both  regular  and  irregular,  globular  clusters 


Fig.  455- 

in  every  state  of  condensation,  and  objects  of  a  nebulous 
character  quite  peculiar,  and  unlike  any  thing  in  other 
regions  of  the  heavens.  In  the  area  occupied  by  the 
nubecula  major  two  hundred  and  seventy-eight  nebulae  and 
clusters  have  been  enumerated,  besides  fifty  or  sixty  outliers, 
which  ought  certainly  to  be  reckoned  'as  its  appendages, 
being  about  six  and  a  half  per  square  degree  ;  which  very 
far  exceeds  the  average  of  any  other  part  of  the  nebulous 
heavens.  In  the  nubecula  minor  the  concentration  of  such 
objects  is  less,  though  still  very  striking.  The  nubeculae, 


ASTRONOMY.  39 1 

then,  combine,  each  within  its  own  area,  characters  which 
in  the  rest  of  the  heavens  are  no  less  strikingly  separated ; 
namely,  those  of  the  galactic  and  the  nebular  system. 
Globular  clusters  (except  in  one  region  of  small  extent)  and 
nebulae  of  regular  elliptic  forms  are  comparatively  rare  in 
the  Milky- Way,  and  are  found  congregated  in  the  greatest 
abundance  in  a  part  of  the  heavens  the  most  remote  possi- 
ble from  that  circle ;  whereas  in  the  nubecube  they  are 
indiscriminately  mixed  with  the  general  starry  ground,  and 
with  irregular  though  small  nebulae. 

THE  NEBULAR  HYPOTHESIS. 

386.  The  Basis  of  the  Nebular  Hypothesis.  —  We  have  seen 
that  the  planets  all  revolve  around  the  sun  from  west  to  east 
in  nearly  the  same  plane,  and  that  the  sun  rotates  on  his  axis 
from  west  to  east.     The  planets,  so  far  as  known,  rotate  on 
their  axes  from  west  to  east;  and  all  the  moons,  except  those 
of   Uranus  and  Neptune,  revolve  around  their  planets  from 
west  to  east.     These  common  features  in  the  motion  of  the 
sun,  moons,  and  planets,  point  to  the  conclusion  that  they  are 
of  a  common  origin. 

387.  Kant's    Hypothesis.  —  Kant,    the    celebrated    German 
philosopher,  seems  to  have  the  best  right  to  be  regarded  as  the 
founder  of  the  modern  nebular  hypothesis.     His  reasoning  has 
been  concisely  stated  thus :  "  Examining  the  solar  system,  we 
find  two  remarkable  features  presented  to  our  consideration. 
One  is,  that  six  planets  and  nine  satellites  [the  entire  number 
then  known]  move  around  the  sun  in  circles,  not  only  in  the 
same  direction  in  which  the  sun  himself  revolves  on  his  axis, 
but  very  nearly  in  the  same  plane.     This  common  feature  of 
the  motion  of  so  many  bodies  could  not  by  any  reasonable 
possibility  have   been  a  result  of   chance :    we   are   therefore 
forced  to  believe  that  it  must  be  the  result  of  some  common 
cause  originally  acting  on  all  the  planets. 

"  On  the  other  hand,  when  we  consider  the  spaces  in  which 
the  planets  move,  we  find  them  entirely  void,  or  as  good  as 
void ;  for,  if  there  is  any  matter  in  them,  it  is  so  rare  as  to  be 


392  ASTRONOMY. 

without  effect  on  the  planetary  motions.  There  is,  therefore, 
no  material  connection  now"  existing  between  the  planets 
through  which  they  might  have  been  forced  to  take  up  a 
common  direction  of  motion.  How,  then,  are  we  to  reconcile 
this  common  motion  with  the  absence  of  all  material  connec- 
tion ?  The  most  natural  way  is  to  suppose  that  there  was  once 
some  such  connection,  which  brought  about  the  uniformity  of 
motion  which  we  observe;  that  the  materials  of  which  the 
planets  are  formed  once  rilled  the  whole  space  between  them. 
There  was  no  formation  in  this  chaos,  the  formation  of  sepa- 
rate bodies  by  the  mutual  gravitation  of  parts  of  the  mass 
being  a  later  occurrence.  But,  naturally,  some  parts  of  the 
mass  would  be  more  dense  than  others,  and  would  thus  gather 
around  them  the  rare  matter  which  rilled  the  intervening  spaces. 
The  larger  collections  thus  formed  would  draw  the  smaller 
ones  into  them,  and  this  process  would  continue  until  a  few 
round  bodies  had  taken  the  place  of  the  original  chaotic 
mass." 

Kant,  however,  failed  to  account  satisfactorily  for  the  motion 
of  the  sun  and  planets.  According  to  his  system,  all  the 
bodies  formed  out  of  the  original  nebulous  mass  should  have 
been  drawn  to  a  common  centre  so  as  to  form  one  sun,  instead 
of  a  system  of  revolving  bodies  like  the  solar  system. 

388.  Herschel's  Hypothesis. —  The  idea  of  the  gradual  trans- 
mutation of  nebulae  into  stars  seems  to  have  been  suggested 
to  Herschel,  not  by  the  study  of  the  solar  system,  but  by  that 
of  the  nebulas  themselves.  Many  of  these  bodies  he  believed 
to  be  immense  masses  of  phosphorescent  vapor :  and  he  con- 
ceived that  these  must  be  gradually  condensing,  each  around 
its  own  centre,  or  around  the  parts  where  it  is  most  dense, 
until  it  should  become  a  star,  or  a  cluster  of  stars.  On  classi- 
fying the  nebulae,  it  seemed  to  him  that  he  could  see  this 
process  going  on  before  his  eyes.  There  were  the  large,  faint, 
diffused  nebulas,  in  which  the  condensation  had  hardly  begun ; 
the  smaller  but  brighter  ones,  which  had  become  so  far  con- 
densed that  the  central  parts  would  soon  begin  to  form  into 
stars ;  yet  others,  in  which  stars  had  actually  begun  to  form ; 
and,  finally,  star-clusters  in  which  the  condensation  was  com- 
plete. The  spectroscopic  revelations  of  the  gaseous  nature  of 


ASTRONOMY.  393 

the  true  nebulas  tend  to  confirm  the  theory  of  Herschel,  that 
these  masses  will  all,  at  some  time,  condense  into  stars. 

389.  Laplace^s  Hypothesis.  —  Laplace  was  led  to  the  nebular 
hypothesis  by  considering  the  remarkable  uniformity  in  the 
direction  of  the  rotation  of  the  planets.  Believing  that  this 
could  not  have  been  the  result  of  chance,  he  sought  to  investi- 
gate its  cause.  This,  he  thought,  could  be  nothing  else  than 
the  atmosphere  of  the  sun,  which  once  extended  so  far  out  as 
to  fill  all  the  space  now  occupied  by  the  planets.  He  begins 
with  the  sun,  surrounded  by  this  immense  fiery  atmosphere. 
Since  the  sum  total  of  rotary  motion  now  seen  in  the  planetary 
system  must  have  been  there  from  the  beginning,  he  conceives 
the  immense  vaporous  mass  forming  the  sun  and  his  atmos- 
phere to  have  had  a  slow  rotation  on  its  axis.  As  the  intensely 
hot  mass  gradually  cooled,  it  would  contract  towards  the  centre. 
As  it  contracted,  its  velocity  of  rotation  would,  by  the  laws  of 
mechanics,  constantly  increase ;  so  that  a  time  would  arrive, 
when,  at  the  outer  boundary  of  the  mass,  the  centrifugal  force 
due  to  the  rotation  would  counterbalance  the  attractive  force 
of  the  central  mass.  Then  those  outer  portions  would  be  left 
behind  as  a  revolving  ring,  while  the  next  inner  portions  would 
continue  to  contract  until  the  centrifugal  and  attractive  forces 
were  again  balanced,  when  a  second  ring  would  be  left  behind ; 
and  so  on.  Thus,  instead  of  a  continuous  atmosphere,  the  sun 
would  be  surrounded  by  a  series  of  concentric  revolving  rings 
of  vapor.  As  these  rings  cooled,  their  denser  materials  would 
condense  first;  and  thus  the  ring  would  be  composed  of  a 
mixed  mass,  partly  solid  and  partly  vaporous,  the  quantity  of 
solid  matter  constantly  increasing,  and  that  of  vapor  diminish- 
ing. If  the  ring  were  perfectly  uniform,  this  condensation 
would  take  place  equally  all  around  it,  and  the  ring  would  thus 
be  broken  up  into  a  group  of  small  planets,  like  the  asteroids. 
But  if,  as  would  more  likely  be  the  case,  some  portions  of  the 
ring  were  much  denser  than  others,  the  denser  portions  would 
gradually  attract  the  rarer  portions,  until,  instead  of  a  ring, 
there  would  be  a  single  mass  composed  of  a  nearly  solid 
centre,  surrounded  by  an  immense  atmosphere  of  fiery  vapor. 
This  condensation  of  the  ring  of  vapor  around  a  single  point 
would  not  change  the  amount  of  rotary  motion  that  had  existed 


394 


ASTRONOMY. 


in  the  ring.  The  planet  with  its  atmosphere  would  there- 
fore be  in  rotation ;  and  would  be,  on  a  smaller  scale,  like  the 
original  solar  mass  surrounded  by  its  atmosphere.  In  the 
same  way  that  the  latter  formed  itself  first  into  rings,  which 
afterwards  condensed  into  planets,  so  the  planetary  atmos- 
pheres, if  sufficiently  extensive,  would  form  themselves  into 
rings,  which  would  condense  into  satellites.  In  the  case  of 
Saturn,  however,  one  of  the  rings  was  so  uniform  throughout, 
that  there  was  no  denser  portion  to  attract  the  rest  around  it; 
and  thus  the  ring  of  Saturn  retained  its  annular  form. 


Fig.  456. 

Such  is  the  celebrated  nebular  hypothesis  of  Laplace.  It 
starts,  not  with  a  purely  nebulous  mass,  but  with  the  sun,  sur- 
rounded by  an  immense  atmosphere,  out  of  which  the  planets 
were  formed  by  gradual  condensation.  Fig.  456  represents 
the  condensing  mass  according  to  this  theory. 

390.  The  Modern  Nebular  Hypothesis.  —  According  to  the 
nebular  hypothesis  as  held  at  the  present  time,  the  sun,  plan- 
ets, and  meteoroids  originated  from  a  purely  nebulous  mass. 
This  nebula  first  condensed  into  a  nebulous  star,  the  star  being 
the  sun,  and  its  surrounding  nebulosity  being  the  fiery  atmos- 
phere of  Laplace.  The  original  nebula  must  have  been  put 
into  rotation  at  the  beginning.  As  it  contracted  and  became 


ASTRONOMY.  395 

condensed  through  the  loss  of  heat  by  radiation  into  space, 
and  under  the  combined  attraction  of  gravity,  cohesion,  and 
affinity,  its  speed  of  rotation  increased ;  and  the  nebulous 
envelop  became,  by  the  centrifugal  force,  flattened  into  a  thin 
disk,  which  finally  broke  up  into  rings,  out  of  which  were 
formed  the  planets  and  their  moons.  According  to  Laplace, 
the  rings  which  were  condensed  into  the  planets  were  thrown 
off  in  succession  from  the  equatorial  region  of  the  condensing 
nebula ;  and  so  the  outer  planets  would  be  the  older.  Accord- 
ing to  the  more  modern  idea,  the  nebulous  mass  was  first  flat- 
tened into  a  disk,  and  subsequently  broken  up  into  rings,  in 
such  a  way  that  there  would  be  no  marked  difference  in  the 
ages  of  the  planets.  The  sun  represents  the  central  portion 
of  the  original  nebula,  and  the  comets  and  meteoroids  its  out- 
lying portion.  At  the  sun  the  condensation  is  still  going  on, 
and  the  meteoroids  appear  to  be  still  gradually  drawn  in  to 
the  sun  and  planets. 

The  whole  store  of  energy  with  which  the  original  solar 
nebula  was  endowed  existed  in  it  in  the  potential  form.  By 
the  condensation  and  contraction  this  energy  was  gradually 
transformed  into  the  kinetic  energy  of  molar  motion  and  of 
heat;  and  the  heat  became  gradually  dissipated  by  radiation 
into  space.  This  transformation  of  potential  energy  into  heat 
is  still  going  on  at  the  sun,  the  centre  of  the  condensing  mass, 
by  the  condensation  of  the  sun  itself,  and  by  the  impact  of 
meteors  as  they  fall  into  it. 

It  has  been  calculated,  that,  by  the  shrinking  of  the  sun  to 
the  density  of  the  earth,  the  transformation  of  potential  energy 
into  heat  would  generate  enough  heat  to  maintain  the  sun's 
supply,  at  the  present  rate  of  dissipation,  for  seventeen  million 
years.  A  shrinkage  of  the  sun  which  would  generate  all  the 
heat  he  has  poured  into  space  since  the  invention  of  the  tele- 
scope could  not  be  detected  by  the  most  powerful  instruments 
yet  constructed. 

The  least  velocity  with  which  a  meteoroid  could  strike  the 
sun  would  be  two  hundred  and  eighty  miles  a  second ;  and 
it  is  easy  to  calculate  how  much  heat  would  be  generated 
by  the  collision.  It  has  been  shown,  that,  were  enough  meteo- 
roids to  fall  into  the  sun  to  develop  its  heat,  they  would  not 


396  ASTRONOMY. 

increase  his  mass  appreciably  during  a  period  of  two  thousand 
years. 

The  sun's  heat  is  undoubtedly  developed  by  contraction  and 
the  fall  of  meteoroids ;  that  is  to  say,  by  the  transformation  of 
the  potential  energy  of  the  original  nebula  into  heat. 

It  must  be  borne  in  mind  that  the  nebular  hypothesis  is  sim- 
ply a  supposition  as  to  the  way  in  which  the  present  solar 
system  may  have  been  developed  from  a  nebula  endowed  with 
a  motion  of  rotation  and  with  certain  tendencies  to  condensa- 
tion. Of  course  nothing  could  have  been  developed  out  of 
the  nebula,  the  germs  of  which  had  not  been  originally  im- 
planted in  it  by  the  Creator. 

IV.     THE    STRUCTURE    OF    THE    STELLAR 
UNIVERSE. 

391.  Sir  William  HerscheTs  View.  —  Sir  William  Herschel 
assumed  that  the  stars  are  distributed  with  tolerable  uniformity 
throughout  the  space  occupied  by  our  stellar  system.  He 


Fig.  457- 

accounted  for  the  increase  in  the  number  of  stars  in  the  field 
of  view  as  he  approached  the  plane  of  the  Milky-Way,  not 
by  the  supposition  that  the  stars  are  really  closer  together  in 
and  about  this  plane,  but  by  the  supposition  that  our  stellar 
system  is  in  the  form  of  a  flat  disk  cloven  at  one  side,  and 
with  our  sun  near  its  centre.  A  section  of  this  disk  is  shown 
in  Fig.  457. 


ASTRONOMY. 


397 


An  observer  near  S,  with  his  telescope  pointed  in  the  direc- 
tion of  Sb,  would  see  comparatively  few  stars  within  the  field 
of  view,  because  looking  through  a  comparatively  thin  stratum 
of  stars.  With  his  telescope  pointed  in  the  direction  Sa,  he 
would  see  many  more  stars  within  his  field  of  view,  even  though 
the  stars  were  really  no  nearer  together,  because  he  would  he 
looking  through  a  thicker  stratum  of  stars.  As  he  directed 
his  telescope  more  and  more  nearly  in  the  direction  Sf,  he 
would  be  looking  through  a  thicker  and  thicker  stratum  of 
stars,  and  hence  he  would  see  a  greater  and  greater  number  of 
them  in  the  field  of  view,  though  they  were  everywhere  in  the 
disk  distributed  at  uniform  distances.  He  assumed,  also,  that 
the  stars  are  all  tolerably 
uniform  in  size,  and  that 
certain  stars  appear  small- 
er than  others,  only  be- 
cause they  are  farther  off. 
He  supposed  the  faint 
stars  of  the  Milky -Way 
to  be  merely  the  most  dis- 
tant stars  of  the  stellar 
disk ;  that  they  are  really 
as  large  as  the  other 
stars,  but  appear  small 
owing  to  their  great  dis- 
tance. The  disk  was  as-  

sumed   to   be    cloven    on  Fis-  458. 

one  side,  to  account  for  the  division  of  the  Milky-Way  through 
nearly  half  of  its  course.  This  theory  of  the  structure  of  the 
stellar  universe  is  often  referred  to  as  the  cloven  disk  theory. 

392.  Hie  Cloven  Ring  Theory.  —  According  to  Madler,  the 
stars  of  the  Milky- Way  are  entirely  separated  from  the  other 
stars  of  our  system,  belonging  to  an  outlying  ring,  or  system 
of  rings.  To  account  for  the  division  of  the  Milky- Way,  the 
ring  is  supposed  to  be  cloven  on  one  side :  hence  this  theory 
is  often  referred  to  as  the  cloven  ring  theory.  According  to 
this  hypothesis,  the  stellar  system  viewed  from  without  would 
present  an  appearance  somewhat  like  that  in  Fig.  458.  The 
outlying  ring  cloven  on  one  side  would  represent  the  stars 


398  ASTRONOMY. 

of  the  Milky-Way;  and  the  luminous  mass  at  the  centre,  the 
remaining  stars  of  the  system. 

393.  Proctor's  View.  —  According  to  Proctor,  the  Milky- 
Way  is  composed  of  an  irregular  spiral  stream  of  minute  stars 
lying  in  and  among  the  larger  stars  of  our  system,  as  repre- 
sented in  Fig.  459.  The  spiral  stream  is  shown  in  the  inner 


Fig.  459- 

circle  as  it  really  exists  among  the  stars,  and  in  the  outer 
circle  as  it  is  seen  projected  upon  the  sky.  According  to  this 
view,  the  stars  of  the  Milky-Way  appear  faint,  not  because 
they  are  distant,  but  because  they  are  really  small. 

394.  Newcomb's  View.  —  According  to  Newcomb,  the  stars 
of  our  system  are  all  situated  in  a  comparatively  thin  zone 
lying  in  the  plane  of  the  Milky- Way,  while  there  is  a  zone 
of  nebulae  lying  on  each  side  of  the  stellar  zone.  He  believes 


ASTRONOMY.  399 

that  so  much  is  certain  with  reference  to  the  structure  of  our 
stellar  universe ;  but  he  considers  that  we  are  as  yet  compara- 


Fig.  460. 

tively  ignorant  of  the  internal  structure  of  either  the  stellar 
or  the  nebular  zones.  The  structure  of  the  stellar  universe, 
according  to  this  view,  is  shown  in  Fig.  460. 


INDEX. 


A. 

Aberration  oflight,  38. 

Aerolites,  304. 

Aldebaran,  star  in  Taurus,  340,  342. 

Algol,  a  variable  star,  343,  358. 

Almanac,  perpetual,  82. 

Alps,  lunar  mountains,  126. 

Altair,  star  in  Aquila,  336. 

Alt-azimuth  instrument,  13. 

Altitude,  12. 

Andromeda  (constellation),  343,  346. 

nebula  in,  376. 

Angstrom's  map  of  spectrum,  164. 
Antares,  star  in  Scorpio,  347. 
Apennines,  lunar  mountains,  122,  124. 
Aphelion,  47. 
Apogee,  44. 
Aquarius,  or  the  Water-Bearer,  350. 

cluster  in,  354. 
Aquila,  or  the  Eagle,  336. 
Arcturus,  star  in  Bootes,  335,  365,  370. 
Argo,  or  the  Ship,  360. 

nebula  in,  383. 

variable  star  in,  360. 
Aries,  or  the  Ram,  350. 
Asteroids,  223,  241. 
Astraea,  an  asteroid,  241. 
Auriga,  or  the  Wagoner,  342. 
Azimuth,  13. 


B. 

Betelgeuse,  star  in  Orion,  340,  370. 
Berenice's  Hair  (constellation),  334. 
Bode's  law,  241. 

disproved,  273. 
Bootes  (constellation),  334,  335. 


c. 

Calendar,  the,  80. 

Callisto,  moon  of  Jupiter,  250. 

Cancer,  or  the  Crab,  350. 

tropic  of,  61. 

Canes   Venatici,   or   the  Hunting-Dogs, 
334- 


Canes  Venatici,  nebula  in,  384. 
Canis  Major,  or  the  Great  Dog,  342. 
Canis  Minor,  or  the  Little  Dog,  340. 
Capella,  star  in  Auriga,  340,  343. 
Capricorn,  tropic  of,  61. 
Capricornus,  or  the  Goat,  350. 
Cassiopeia  (constellation),  332. 

new  star  in,  362. 
Castor,  star  in  Gemini,  340,  370. 
Caucasus,  a  lunar  range,  124. 
Centaurus,  star-cluster  in,  355. 
Cepheus  (constellation),  334. 

nebula  in,  387. 
Ceres,  the  planet,  241. 
Cetus,  or  the  Whale,  346. 

variable  star  in,  359. 
Charles's  Wain,  330. 
Circles,  great,  4. 
diurnal,  8. 
hour,  16. 
small,  4. 
vertical,  12. 
Clock,  astronomical,  18. 

time,  78. 

Coma  Berenices,  or  Berenice's  Hair,  334. 
Comet,  Biela's,  293. 

and  earth,  collision  of,  316. 
Coggia's,  297. 
Donati's,  296. 
Encke's,  293. 
Halley's,  291. 
of  1680,  290. 
of  1811,  290. 
of  1843,  295. 
of  1861,  297. 
of  June, 1881,  300. 
Comets,  appearance  of,  274. 
and  meteors,  313. 
bright,  274. 

chemical  constitution  of,  318. 
development  of,  277. 
number  of,  288. 
orbits  of,  282. 
origin  of,  287. 
periodic,  286. 

physical  constitution  of,  314. 
tails  of,  279. 
telescopic,  275,  281. 
visibility  of,  281. 
Conic  sections,  48. 

401 


402 


INDEX. 


Conjunction,  91. 

inferior,  130. 

superior,  130,  136. 
Constellations,  325. 

zodiacal,  32. 

Copernican  system,  the,  44,  53. 
Copernicus,  a  lunar  crater,  120,  129. 
Corona  Borealis,  or  the  Northern  Crown, 

336. 

Corona  Borealis,  new  star  in,  363. 
Corvus,  or  the  Crow,  339. 
Crystalline  spheres,  41. 
Cycles  and  epicycles,  42. 
Cygnus,  or  the  Swan,  338. 

D. 

Day  and  night,  57. 

civil,  77. 

lunar,  108. 

sidereal,  74. 

solar,  74. 
Declination,  16. 
Deimos,  satellite  of  Mars,  239. 
Delphinus,  or  the  Dolphin,  338. 
Deneb,  star  in  Cygnus,  338,  370. 
Dione,  satellite  of  Saturn,  259. 
Dipper,  the  Great,  330,  366,  369,  370. 
the  Little,  331. 
the  Milk,  347. 
Dissociation,  163. 
Dominical  Letter,  the,  81. 
Draco,  or  the  Dragon,  331. 


E. 

Earth,  density  of,  85. 

flattened  at  poles,  55. 
form  of,  53. 
in  space,  56. 
seen  from  moon,  109. 
size  of,  55. 
weight  of,  83. 
Eccentric,  the  43. 
Eccentricity,  46. 
Eclipses,  210. 

annular,  219. 
lunar,  210,  214. 
solar,  216. 
Ecliptic,  the,  27. 

obliquity  of,  28. 
Ellipse,  the  45,  49. 
Elongation,  of  planet,  130. 
Enceladus,  moon  of  Saturn,  259. 
Epicycles,  42. 
Epicycloid,  107. 

Epsilon  Lyrae,  a  double  star,  356. 
Equator,  the  celestial,  7. 
Equinoctial,  the,  7. 

colure,  16. 
elevation  of,  9. 
Equinox,  autumnal,  29. 
vernal,  16,  29. 

Equinoxes,  precession  of,  31,  85. 
Eta  Argus,  a  variable  star,  360,  383. 
Europa,  moon  of  Jupiter,  250. 


F. 


Faculse,  solar,  177. 
Fomalhaut,  star  in  Southern 
Fraunhofer's  lines,  164,  371. 


Fish,  350. 


Galaxy,  the,  326. 
Ganymede,  moon  of  Jupiter,  250. 
Gemini,  or  the  Twins,  340. 
Georgium  Sidus,  271. 


H. 

Hercules  (constellation),  336. 

cluster  in,  353. 

orbit  of  double  star  in,  357. 

solar  system  moving  towards, 

367. 

Herschel,  the  planet  (see  Uranus). 
Herschel's  hypothesis,  392,  396. 
Horizon,  the,  5. 
Hyades,  the,  342,  350. 
Hydra,  or  the  Water-Snake,  340. 
Hyperbola,  the,  49. 
Hyperion,  moon  of  Saturn,  259. 


lo,  moon  of  Jupiter,  250. 
Irradiation,  90,  113. 


J- 

Japetus,  moon  of  Saturn,  259. 
Job's  Coffin  (asterism),  338. 
Juno,  the  planet,  241. 
Jupiter,  apparent  size  of,  245. 

distance  of,  245. 

great  red  spot  of,  249. 

orbit  of,  244. 

periods  of,  246. 

physical  constitution  of,  246. 

rotation  of,  248. 

satellites  of,  250. 

eclipses  of,  252. 
transits  of,  254. 

volume  of,  245. 

without  satellites,  255. 


K. 


129. 


Kant's  hypothesis,  391. 
Kepler,  a  lunar  crater,  u 
Kepler's  system,  44. 

laws,  46. 

star,  362. 
Kirchhoff's  map  of  spectrum,  164. 


INDEX. 


403 


L. 

Moon,  orbital  motion  of,  91. 

phases  of,  93. 

Laplace's  hypothesis,  392. 
Latitude,  celestial,  30. 

real  size  of,  88. 
rising  of,  99. 

Leap  year,  81. 
Leo,  or  the  Lion,  334. 
Leonids  (meteors),  312. 
Libra,  or  the  Balances,  347. 

rotation  of,  102. 
sidereal  period  of,  92. 
surface  of,  115. 
synodical  period  of,  92. 
terminator  of,  115. 

Longitude,  celestial,  30. 

wet  and  dry,  98. 

Lyra,  or  the  Lyre,  338. 

double  star  in,  356. 

N. 

M 

Nadir,  the,  6. 

1V1. 

Neap-tides,  72. 

Magellanic  clouds,  the,  389. 
Magnetic  storms,  190. 
Magnetism  and  sun-spots,  190. 
Mars,  apparent  size  of,  236. 
brilliancy  of,  237. 
distance  of,  235. 
orbit  of,  235. 
periods  of,  237. 
rotation  of,  239. 
satellites  of,  239. 
volume  of,  236. 
Mercury,  apparent  size  of,  226. 
atmosphere  of,  228. 

Nebula,  in  Andromeda,  376. 

dumb-bel'l,  383. 
in  Argus,  383. 
in  Canes  Venatici,  384. 
in  Cepheus,  387. 
in  Orion,  378. 
in  the  Triangle,  387. 
in  Ursa  Major,  386. 
Nebula;,  281,  330,  373. 
annular,  373. 
circular,  373. 
condensation  of,  385. 

distance  of,  225. 
elongation  of,  227. 
orbit  of,  225. 
periods  of,  227. 
volume  of,  226. 
Meridian,  the,  12. 
Meridian  circle,  17. 
Meridians,  celestial,  31. 
Meteoric  iron,  305,  307. 
showers,  310. 
stones,  305. 

elliptical,  373. 
irregular,  376. 
multiple,  375. 
spiral,  373,  384. 
Nebular  hypothesis,  the,  391. 
Neptune,  discovery  of,  271. 
orbit  of,  271. 
satellite  of,  274. 
New  style,  80. 
Newcomb's  theory  of  the  stellar  universe, 

«.,Q 

Meteors,  300. 
August,  311. 
light  of,  309. 
November,  312. 
sporadic,  310. 
Meteoroids,  308. 

39°- 
Newton's  system,  48. 
Nodes,  97. 
Nubecula,  Major,  389. 
Minor,  389. 
Nutation,  34. 

Micrometers,  20,  153. 

Milky-  Way,  the,  326. 

Mimas,  moon  of  Saturn,  259. 

• 

Mira,  a  variable  star,  359. 
Moon,  apparent  size  of,  87,  89. 
aspects  of,  91. 
atmosphere  of,  109. 

Olbers's  hypothesis,  241. 
Old  style,  80. 
Ophiuchus  (constellation),  347. 

chasms  in,  123. 

new  star  in,  362. 

craters  in,  119. 

Opposition,  91,  136. 

day  of,  108. 

Orion,  341. 

distance  of,  86. 
eclipses  of,  210. 

nebula  in,  378. 
the  trapezium  of,  356. 

form  of  orbit,  97. 

harvest,  101. 

hunter's,  102. 

P. 

inclination  of  orbit,  97. 

kept  in  her  path  by  gravity,  51. 
librations  of,  102. 

Pallas,  the  planet,  241. 
Parabola,  the,  49. 

mass  of,  90. 

Parallax,  37. 

meridian  altitude  of,  98. 
mountains  of,  116. 

Pegasus  (constellation),  343,  346. 
triple  star  in,  356. 

404 


INDEX. 


Perigee,  44. 
Perihelion,  47. 
Perseids  (meteors),  311. 
Perseus  (constellation),  346. 

cluster  in,  353. 

Phobos,  satellite  of  Mars,  239. 
Pico,  a  lunar  mountain,  127. 
Pisces,  or  the  Fishes,  350. 
Piscis  Australis,  or  the  Southern  Fish, 

350. 
Planets,  39. 

inferior,  130. 

periods  of,  132. 
phases  of,  132. 

inner  group  of,  221. 

intra-Mercurial,  230. 

minor,  223. 

outer  group  of,  222,  244. 

superior,  134. 

motion  of,  134. 
periods  of,  137. 
phases  of,  137. 

three  groups  of,  221. 
Pleiades,  the,  328,  342,  351. 
Pointers,  the,  330. 
Polar  distance,  16. 
Pole  Star,  the,  7,  330,  365. 
Poles,  celestial,  7,  9. 
Pollux,  star  in  Gemini,  340,  370. 
Praesepe,  or  the  Beehive,  350. 
Precession  of  equinoxes,  31,  85. 
Prime  vertical,  the,  12. 
Proctor's  theory  of  the  stellar   universe, 

398. 

Procyon,  star  in  Canis  Minor,  340. 
Ptolemaic  system,  the,  41. 


Q- 

Quadrature,  91,  137. 

R. 

Radiant  point  (meteorsV^io. 
Radius  vector,  47. 
Refraction,  35. 

Regulus,  star  in  Leo,  334,  370. 
Rhea,  moon  of  Saturn,  259. 
Rigel,  star  in  Orion,  340,  370. 
Right  ascension,  16. 

S. 

Sagittarius,  or  the  Archer,  347. 
Saturn,  apparent  size  of,  256. 
distance  of,  256. 
orbit  of,  255. 
periods  of,  256. 
physical  constitution  of,  257. 
ring  of,  261. 

changes  in,  268. 

constitution  of,  269. 

phases  of,  263. 


Saturn,  rotation  of,  258. 

satellites  of,  259. 

volume  of,  256. 
Scorpio,  or  the  Scorpion,  347. 

cluster  in,  355. 
Seasons,  the,  64. 
Sirius,  the  Dog-Star,  .340,  342,  365,   370, 

c  .   37I- 

Solar  system,  the,  41. 

Solstices,  29,  59,  60. 

Sound,  effect  of  motion  on,  168. 

Spectra,  bright-lined,  158. 

comparison  of,  154. 
continuous,  158. 
displacement  of  lines  in,  171. 
of  comets,  318. 
reversed,  161. 
sun-spot,  193. 
types  of  stellar,  371. 
Spectroscope,  the,  152. 

diffraction,  157. 
direct-vision,  155. 
dispersion,  152. 
Spectrum  analysis,  159. 

solar,  164. 
Sphere,  defined,  3. 

the  celestial,  5. 

rotation  of,  7. 
Spring-tides,  72. 
Stars,  circumpolar,  7. 

clusters  of,  328,  350. 
color  of,  357. 
constellations  of,  325. 
constitution  of,  371. 
distance  of,  364. 
double,  355. 
drift  of,  368. 
four  sets  of,  10. 
magnitude  of,  322. 
motion  of,  in  line  of  sight,  369. 
multiple,  356. 
names  of,  325. 
nebulous,  373. 
new,  361. 
number  of,  323. 
parallax  of,  364. 
proper  motion  of,  365. 
secular  displacement  of,  366. 
temporary,  361. 
variable,  358. 
Sun,  atmosphere  of,  149. 
brightness  of,  151. 
chemical  constitution  of,  164. 
chromosphere  of,  149,  196. 
corona  of,  149,  196,  204. 
distance  of,  142. 
faculae  of,  177. 
heat  radiated  by,  150. 
inclination  of  axis  of,  187. 
mass  of,  140. 

motion  of,  among  the  stars,  26. 
at  surface  of,  168. 
in  atmosphere  of,  172. 
secular,  366. 

photosphere  of,  149,  175. 
prominences  of,  149,  197. 
rotation  of,  186. 


INDEX. 


405 


Sun,  spectrum  of,  164,  171. 
temperature  of,  149. 
volume  of,  140. 
winds  on,  174. 
Sun-spots,  179. 

and  magnetism,  190. 

birth  and  decay  of,  185. 

cause  of,  194. 

cyclonic  motion  in,  182. 

distribution  of,  188. 

duration  of,  181. 

groups  of,  181. 

periodicity  of,  189. 

proper  motion  of,  187. 

size  of,  181. 

spectrum  of,  193. 


T. 

Taurus,  or  the  Bull,  342. 

quadruple  star  in,  356. 
Telescope,  Cassegrainian,  23. 

equatorial,  19. 

front-view,  22. 

Gregorian,  23. 

Herschelian,  22. 

Lord  Rosse's,  25. 

Melbourne,  25. 

Newall,  20. 

Newtonian,  22. 

Paris,  26. 

reflecting,  21.  , 

Washington,  20. 

Vienna,  20. 

Telespectroscope,  the,  155. 
Telluric  lines  of  spectrum,  165. 
Tethys,  moon  of  Saturn,  259. 
Tides,  67. 
Time,  clock,  78. 

sun,  78. 

Titan,  moon  of  Saturn,  259,  261. 
Toucan,  star  cluster  in,  354,  389. 
Transit  instrument,  17. 
Transits  of  Venus,  145. 
Triesneker,  lunar  formation,  123. 
Tropics,  61. 
Twilight,  62. 
Tycho  Brahe's  star,  361. 

system,  44. 
Tycho,  a  lunar  crater,  129. 


U. 

Universe,  structure  of  the  stellar,  396. 
Uranus,  discovery  of,  271. 

name  of,  270. 

orbit  of,  269. 

satellites  of,  271. 
Ursa  Major,  or  the  Great  Bear,  330. 

nebula  in,  386. 
Ursa  Minor,  or  the  Little  Bear,  330. 


V. 

Vega,  star  in  Lyra,  336,  365,  370. 
Venus,  apparent  size  of,  231. 

atmosphere  of,  234. 

brilliancy  of,  232. 

distance  of,  231. 

elongation  of,  231. 

orbit  of,  230. 

periods  of,  232. 

volume  of,  231. 

transits  ot,  145,  234. 
Vernier,  the,  15. 
Virgo,  or  the  Virgin,  338. 
Vesta,  the  planet,  241. 
Vulcan,  the  planet,  230. 


Y. 

Year,  the,  78. 

anomalistic,  79. 
Julian,  80. 
sidereal,  79. 
tropical,  79. 


Zenith,  the,  6. 

distance,  12. 
Zodiac,  the,  32. 
Zodiacal  constellations,  32. 

light,  318. 
Zones,  61. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
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